Hill Dwarf Eldritch Knight is a good option

Classes to consider

Wisdom based classes include Cleric, Druid, Ranger, and Monk.

These have hit dice of d8, except ranger that has a d10.

Int means you'd be looking at Wizard (d6 hit die).

However, you could pick Fighter / Eldritch Knight which makes good use of your decent Strength/Dexterity for combat and your great Intelligence for spellcasting support. That would also give you an extra ASI at level 6. That means two ASIs available to boost Con by 2 each time.

So your base HP would be 6 for Cleric/Druid/Ranger, or 12 for Hunter, or 18 for Fighter.

Ways to pickup Con

Hill Dwarf (PHB page 20) grants +2 Con and +1 Wisdom. That reduces the penalty to the hit die at each level by 1 so, after six levels, the net result would be +6 HP. Plus you get +6 HP from Dwarven Toughness for a 6th level character (and a +1 bump to your Wisdom mod, for what it's worth).

Fire Genasi (if you're allowed to choose the race) would be almost the same except that your Int is on the even stat value. Bumping Int by 1 would not increase the mod.

Taking the Tough feat would net you 12 HP compared to 6HP from a straight +2 to Con, so it would be wiser to pick the feat at level 4.

If you pick the Eldritch Knight and get an extra ASI at level 6, you could boost your Con again to a net 48 maximum possible HP (18 base, +12 from racial boost, +12HP from ASI @ L4, +6HP from ASI @ L6).

With rolled HP increases, you'd probably end up around half that at 24ish hot points.

Other perks to Eldritch Knight

Eldritch Knight would grant you access to heavy armor, Defense Fighting Style, Second Wind, and the Shield spell (+5 AC as a reaction) meaning that you would avoid a LOT of damage simply by not getting hit in the first place.

Given the variability inherent in rolling stats, boosting your AC to high levels becomes extremely valuable.

It's going to take some work, but you can make this survivable. You're going to be a Variant Human with the Tough feat. If you give yourself +1 Con, combined with the Tough feat, your hp penalty will drop to -1 per level. At that point, you could make most builds work, but there are a couple I'd like to suggest:

Barbarian 1/Moon Druid 2/Old One Warlock 1/Rogue 1

This is probably my favourite, but it toes the line of the limitations on repeating classes - while 2 levels is normally a dip, this build has more levels in Druid than any of their other characters, and it's the focus of the build. If you can get away with it, here's how it works.

Starting with Barbarian gets you a nice starting hp of 11. Next, you're dropping 2 levels into Druid so you can turn into a brown bear. That's the idea here - you are a bear. While your hp total as a human shouldn't be completely terrible, the aim is that you will be a bear every time you are in combat.

Your Barbarian level lets you boost a brown bear's AC from 11 to 13, as well as Rage, which gives you resistance to mundane physical damage and a minor damage boost. Old One Warlock gives you the telepathy you need to communicate with your party members, as well as some short rest spell slots you can use to heal yourself while in bear form.

After that, you're taking a Rogue level, so that you can pick up Expertise in your Perception and Insight skills. The idea here is to be as difficult as possible to surprise, because surprise is deadly for you - you can't afford to be caught in your human form.

The last level is up to you - none of the level 2 features of the Barbarian, Rogue, or Warlock stand out to me. One kinda nice option is to take a Monk level - if you use your other +1 from Variant Human on Wisdom, the Monk's Unarmoured Defence will boost your bear AC up to 14. It's not a huge bonus over the Barbarian's, but you can use every bit you can get.

Bear Totem Barbarian 3/Moon Druid 2/Old One Warlock 1

This is a variation on the above build - you're now playing a Variant Human, which puts your human form hp back in the toilet, but nets you the Alert feat, so you're never caught unawares. Ditching the Rogue level lets you take Barbarian all the way to 3, which gives you the (wildly appropriate) Bear Totem, extending your resistances from Rage to all damage except psychic.

Fighter 1/Forge Cleric 4/Wizard 1

And now for something completely different - this is the "my hp sucks, but it (hopefully) never matters" build. Starting with a Fighter level gets you a bit more hp (9 instead of 7), the Defense Fighting Style, for +1 AC, and proficiency in Con saving throws, meaning you have at least some chance of keeping your concentration spells going.

After that, you're just like any other Cleric, except that you have 22 AC in plate armor and a shield, instead of 20. The shield of faith spell brings that up to 24 AC when you really don't want to get hit. We top it off with a Wizard level, which gets you two important emergency button spells: The first, as always, is shield, for +5 AC, when you really don't want to get hit. The second is absorb elements, for when people attack you in ways that don't involve your AC.

This build has a gaping flaw, of course - attacks that don't target AC, and deal radiant, necrotic, or psychic damage. My recommendation is to stand near a friendly Paladin.

You probably shouldn't play a main caster

You want to make use of your high Intelligence and Wisdom scores, but playing a caster is not only about these scores. You want a non-terrible Constitution score in order to maintain concentration on your spells. For you this will be all but impossible as you would have to roll a 15 every time you take any damage (you'd have to roll higher if you take more damage, but with so little HP from the spellcaster classes, more damage would sooner kill you).

As such, the use of these scores should be as secondary in a different class. The best classes for this would be Arcane Trickster or Eldritch Knight (to make use of the Intelligence) or a Cleric without focusing on Concentration spells (to make use of the Wisdom).

Rykara's answer is an excellent showing of how to mitigate the HP-loss with your race, class, and feats. I don't think a cleric without Concentration spells would be more effective than his fighter for this, so I'll touch on ho to build an Arcane Trickster (with the intent to avoid taking damage as opposed to being a hair more bulky).

The slippery Arcane Trickster

The rogue offers a number of options that increase your ability to avoid damage.

1. Cunning Action gives you the option to Disengage or Hide as a bonus action allowing you to exit an unfortunate melee without provoking an opportunity attack, and in the right situation gain the benefits of Unseen Attackers which as a start gives enemies disadvantage when attacking you, but also lets you leverage Sneak Attack for heavy damage.


2. Expertise in Stealth improves that Hide ability greatly making even the perceptive creatures have trouble tracking you on a battlefield with sufficient cover.


3. Uncanny Dodge comes in as a fail-safe so that even if an attack does hit you, you only take half damage.


4. Your next level gives you Evasion which is very good at avoiding that pesky AOE spell damage even when you succeed on the saving throw.


5. Arcane Trickster gives you access to the very useful shield spell 3 times a day (or other utility spells if you don't find yourself in harrowing situations enough to require this).

Dragonmark of Shadow

Even with the benefits of an Arcane Trickster defensively, you do not want to be in the fray very often. As such, I recommend a ranged weapon build, this means maximizing Dexterity, and one of the Dexterity increasing races is actually very good for your build.

The Mark of Shadow elf, in Wayfinder's Guide to Eberron, gives you an extra d4 to Stealth checks (which on top of Expertise makes you very hard to detect), and it also lets you use the Hide action even without cover once per rest:

You can use the Hide action as a bonus action, even if you have no cover or if you’re under observation. Regardless of whether you succeed or fail, once you use this ability, you can’t use it again until you finish a short or long rest.

This can save your life in situations where cover is not readily available and you've run out of shield slots (or if you're up against a spellcaster, who can get around your armor class, that you really don't want to know your location).

Elf also puts your Dexterity at 17, allowing you to catch up with other weapon characters (who will have 18 in their primary attack stat) using your level 4 ability score increase.

The build

Here is a sample finalized build using the above advice:

• Level 1:
  
  
  
  
  • Mark of Shadow Elf
  
  
  • Proficiency in Stealth, Sleight of Hand (Mage Hand Legerdemain), Perception (high Wisdom), and Investigation (high Intelligence) from Rogue
  
  
  • Proficiency in two knowledge Skills (i.e. Religion, Nature) relevant to the campaign from background (to use that Intelligence)
  
  
  • Note: all of your skill-relevant ability scores are passible, so with Expertise, you can pick and choose what skills you think will be the most useful
  
  
  • Expertise in Stealth and Perception (these are your two most useful skills in my experience, although another option can be taken instead of Perception depending on how often your table uses skills like Investigation and the knowledge skills)
  
  
  
  


• Level 2:
  
  
  
  
  • Nothing really important to choose here
  
  
  
  


• Level 3:
  
  
  
  
  • Arcane Trickster, of course. make sure you pick up the shield spell, some utility spells that utilize your Intelligence (like disguise self or tasha's hideous laughter), and maybe a cantrip that helps you deal extra damage in some situations (like sword burst/thunderwave if you are surrounded or booming blade if you are willing to risk going into melee range)
  
  
  
  


• Level 4:
  
  
  
  
  • An ability score increase gets your Dexterity to 18 and brings your Wisdom to 18 or Constitution to 4 (depending on whether you want 6 more HP or better Perception)
  
  
  • Alternatively, the Greater Dragonmark feat will give you a, extra d6 instead of a d4 for your Stealth and Deception checks, and gives you some powerful spells while still bringing your Dexterity to 18 (this would be my choice).
  
  
  • If you expect to have cover a lot (with which to be hidden), sharpshooter is probably the best. Since you have advantage as an Unseen Attacker, you can capitalize on the +10 damage often and you can also be further away from your enemies by attacking from long range.
  
  
  
  


• Level 5:
  
  
  
  
  • Nothing really important to choose here
  
  
  
  


• Level 6:
  
  
  
  
  • 2 more expertise to put in skills of your choice (again, your knowledge skills could be very useful, but just about any skill is viable as most of your ability scores are pretty solid)
  
  
  
  


• Vital Equipment:
  
  
  
  
  • You want to have as high an Armor Class as possible so try your best to get Studded Leather Armor.
  
  
  • The best ranged weapon to be using is a Light Crossbow.
  
  
  • You may want to have a Rapier on hand in case the opportunity for booming blade or green-flame blade presents itself (remember, a light crossbow only takes up to hands when firing, so you can draw a rapier as part of your attack, and stow it before you attack with the light crossbow in later turns)
  
  
  • You want a component pouch so you can cast spells like Tasha's Hideous Laughter, or just ask your GM if you can have the components listed in each Material spell you have (again, you can hold the light crossbow in one hand while not attacking allowing you to use Somatic spells while still hanging on to your weapon)
  
  
  
  

An Abjurer can get some staying power without constitution

An abjuration Wizard gains the Arcane Ward feature at 2nd level, which provides twice your level plus your intelligence modifier in extra hit points that replenish as you cast abjuration spells. At 6th level with your base stats this is at least 16 extra maximum hit points. That's a massive improvement over the measly 8ish max hp your wizard would otherwise probably have!

Being a wizard also lets you ameliorate the effects of your low constitution a bit: while your d6 hit die will almost never outweigh your negative Constitution modifier, your minimum HP gain per level can't go below 1. This is particularly good since you are, for some reason, focused on median rather than mean hit points-- while your mean or expected hp at level 6 from wizard levels with 3 Con is 7.5, your median hp is a whopping 2 points higher: 9.5. This is because there are a very limited selection of values your hp can be, and there's about an 80% chance it's one of the bottom two.

A Warlock can charge Arcane Wards all day, every day

Warlocks get the only unlimited at-will spellcasting ability we can use to fuel Arcane Ward: the Armor of Shadows eldritch invocation. This lets us "cast an abjuration spell of first level or higher" an unlimited number of times without resting, plus we get to up our AC a bit. It's only 2 hp per casting, so it'll take 48 seconds to fully recharge a downed ward, but if you need your ward back up faster than that you're better off casting one of them fancy higher level spells anyways.

It's two levels to get invocations, so you'll be War2/Wiz4. Eventually, in the far future, you'll really want to replace those levels in Warlock, but that's at level 18, which is far enough out I think it's a worthwhile trade-off.

Your levels in Warlock also give you a patron, which should be either the Raven Queen or the Undying.

The first gives you access to the Sanctuary spell, which is a nice effect for a support caster, and also is an abjuration so it'll recharge your ward a tiny bit when used, as well as a cool raven familiar that stacks with other familiars you might have except that it's totally not a problem if it dies repeatedly. The raven is particularly good since it boosts your perception score (by 2 in your case) so you are less likely to get one-shotted by an unexpected ambush.

The Undying instead gives you a sanctuary-spell-like effect that's always on, but only functions against undead and only until they first make the save. Tomb of Anhinnialation has a lot of undead baddies, so this is pretty comparable, but that's all it's going to give you. Your call on whether saving the allowed-to-cast-spells-this-round flag for something else is worth it.

Note that Warlocks have a d8 HD so you suffer more from your negative Con score: a regular character would gain around 1 more hp per level from that change but you only gain around .58 hp per level more over those two levels. Our expected hp and median are still a bit higher this way, of course, and we end up with an expected hp of 10.42 and a median hp of 12.5.

Our second invocation can be utility stuff like Eyes of the Rune-Keeper or Eldritch Sight, or 8 more hit points via false life via Fiendish Vigor.

Half-orcs get a do-over

Half-orcs (and Minotaurs from Planescape:Amonkhet, but those swap out darkvision for a garbage natural weapon) get +1 constitution and the ability to instead be fine the first time they would be reduced to 0 hp or below each long rest. This doesn't trigger if you are instakilled by massive damage, which is a significant concern for you since that's, like, 45 damage and there are CR 6 creatures that can actually do that in a single round if you let them (don't get charge-stomped by a mammoth).

The final recommended build would look something like this:

Half-Orc War2/Wiz4

AC 15

hp: 12

temp hp: 8

arcane ward: 17

Str: 16

Dex: 15

Con: 4

Int: 20

Wis: 17

Cha: 13

Background: Stojanow Prisoner

Skills: Perception, Deception, Investigation, Arcana, Intimidation

Tools: Thieves' Tools, Gaming Set

Patron: Raven Queen

Invocations: Armor of Shadows, Fiendish Vigor

Warlock Cantrips:

Eldritch Blast, Minor Illusion

Pact Magic Spells Known:

Sanctuary, Armor of Agathys, Hellish Rebuke

Wizard Cantrips:

Control Flame, Encode Thoughts, Shape Water, Blade Ward

Wizard Spells Known:

Comprehend Languages, Identify, Fog Cloud, Tenser's Floating Disk, Find Familiar, Grease, Unseen Servant, Lonstrider (+ whatever you can get from scrolls, NPCs, etc)

Flock of Familiars, Detect Thoughts, Rope Trick, Enlarge/Reduce (+ etc)

Out of combat, you are fairly tanky since your hp completely replenish after each trap, fall, firewalking exercise, or what have you.

In combat, you want to be in the back and avoid damage as much as possible, using an unseen servant and/or flock of familiars and/or your sentinel raven to control enemy movement options. You can cast enlarge or false life on an ally to buff them without losing sanctuary, and you can cast sanctuary twice per short rest to discourage enemies from attacking you. Armor of Agathys can singlehandedly eliminate hordes of weaker enemies who attack you, because your Arcane Ward will ensure that the temp hp from that spell stay around until you're basically dead anyways. Eldritch Blast provides an option to deal okayish ranged damage if it comes up, and something to do while sanctuarying when defence isn't urgent enough to warrant a blade ward.

Out of combat, your wide variety of utility spells, massive perception abilities, and proficiency in Thieves' Tools will let you act a bit like a rogue.

Moon Druid might be your best option

The feature of all Druids, irrespective of their archetype, is their ability to gain a "shield" of hp through their wild shape forms. Because Moon Druids are incentivized to shapeshift a lot, and they gain access to very powerful wild shape forms, being able to at-will shapeshift into a high hp creature (with some decent attacks to accompany them) will be very beneficial in keeping you alive. You'll also gain access to some powerful spellcasting abilities, and your high Wisdom score will complement those abilities.

At level 6, you are limited to CR2 Beasts. So you could shapeshift into, say, a Saber-toothed Tiger, gaining 52 hitpoints. Or you could shapeshift into a Giant Constrictor Snake, gaining 60 hit points. You have a lot of ways to substantially improve your survivability by simply relying on the extra hitpoints provided by these kinds of forms, and combined with your ability to also regenerate hitpoints either through Cure Wounds or your Combat Wild Shape feature, you'll offset the lost Constitution by a tremendous factor.

Another consideration is the fact that while shapeshifted, you only retain your mental stats (Intelligence, Wisdom, Charisma), whereas your other stats are replaced by the form you choose. So in a wild shape form, your lack of Constitution might as well not exist.

There are limitations of course: if you ever fall out of those forms, you'll be vulnerable. But that would be a problem regardless, and I think this shores up your weakness in a reliable manner.

Consider Lizardfolk to further bolster survivability

Irrespective of whether you go Moon Druid or not, gaining the extra +2CON is a good way to at least recover some of those lost maximum hitpoints, and since it also provides a +1 to Wisdom, it'll at least complement your class choice.