Fixed mypy type inferrence for TypeAdapter

[zakstucke]

Change Summary

Add __init__ TYPE_CHECKING block to TypeAdaptor similar to one for __new__.

Related issue number

fix #6615

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin

[hramezani]

please review

[adriangb]

I used the following little script to test:

from pydantic import TypeAdapter
from typing_extensions import reveal_type

reveal_type(TypeAdapter(int))
reveal_type(TypeAdapter(int | str))

Then mypy test.py:

test.py:4: note: Revealed type is "pydantic.type_adapter.TypeAdapter[builtins.int]"
test.py:5: note: Revealed type is "pydantic.type_adapter.TypeAdapter[types.UnionType]"

Or pyright test.py:

test.py:4:13 - information: Type of "TypeAdapter(int)" is "TypeAdapter[int]"
test.py:5:13 - information: Type of "TypeAdapter(int | str)" is "TypeAdapter[int | str]"

So this seems fine to me! Not sure why I went with __new__ originally, there was a reason, but this works so 🤷🏻‍♂️