If a character gains a positive feature that makes them more resilient to Stress such as "Durable" so that when he/she is hit in combat he/she resists damage by 1 pt, would you also rules that this makes them resistant to damage the character would normally accrue from leftover negative dice in the dice pool for the physical track? I am speaking of leftover negative dice in the dice pool from performing tasks in-game that don't get canceled out.

In addition, zombies inherently have resistance 1 against stress. Do you count this resistance against the same kind of uncanceled negative dice in the dice pool?

I think RAW, Features can only add dice to checks, so this is completely a creation of your own doing and therefore only you can decide what is right for it. That said, Resistance is a VERY strong mechanical effect (far stronger than 1, or even 2 bonus dice depending on the associated Attributes), and accordingly I would advise you to only apply it to damage/dealt Stress if that is how you envisioned it (decrease its disbalanced effect), however as a mechanic Resistance is supposed to apply to any Stress taken in that category, so...

Do you think allowing a player to have a positive feature such as that is OP? Should I make him change it?

I do think it's OP if it acts as normal Resistance, because that makes it effectively an automatic cancel (and therefore, an automatic success) of a negative die's main purpose (to inflict Stress). Whereas normal Features just add die which are only an increased chance of success.

If it just reduces damage from Melee attacks, I think that's probably A-OK, but if it reduces damage from any Physical attack (i.e. Firearms), i think that's probably still a bit OP (not to mention unrealistic) too. Ultimately I think it would be best to stick to the "Posi-features add positive dice" model, which you could still do this same Feature with, it would just only apply to defensive (say, resisting Stress from a fall, or long run, or other tests where the damage is not coming from an offensive attack) and healing tests.

All that said, I was contemplating having similar Features (+1 damage to Melee or Firearm attacks, +1 Resistance, +1 Trauma "slot", etc.) as "capstone" achievements for progressing along a certain Feature path, when I was working on my Homebrew advancement system, but I decided to not try something so major until I was more familiar with the game.

I have a semi related question on resistance from stress, when the stress is mitigated (ie. to trauma)I assume the resistance to said type attack goes away and a vote for a new feature demonstrating the character's consistant exposure to that stress can be done during advancement, in the for of a positive die? It's a bit vague, but as you said resistace is a big thing,can't see it being permanent myself, but could see a positive feature coming from it in play during advancement.