Support Sentinel types in pydantic models

[MartinBernstorff]

Describe the Bug

import pydantic
from pydantic.experimental.missing_sentinel import MISSING

class UpdateUserPayload(pydantic.BaseModel):
    phone_number: str | MISSING = MISSING 
    # Errors with "Expected a type form, got instance of `type[str] | UnionType`". Should not error.

This is an experimental feature, but required if you have a PATCH model and the generated schema shouldn't erroneously indicate that the value is nullable.

https://pydantic.dev/docs/validation/latest/concepts/experimental/#missing-sentinel

Sandbox Link

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(Only applicable for extension issues) IDE Information

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[MartinBernstorff]

This depends on a PEP which has just been approved: https://peps.python.org/pep-0661/

[hrolfurgylfa]

I'm doing some experimentation for this on my branch here: https://github.com/hrolfurgylfa/pyrefly/tree/PEP-661-Sentinel

Still a decent bit off from working, but I think I have the wiring working all the way for the new Sentinel type, just need to correct it and add more tests. Also clean up my TODOs I've left along the way.

Aiming to have the first test passing tomorrow, but the code won't be ready to submit for a review for a few days at least, maybe a week or so.