[flake8-pyi] Fix false positive for PEP 695 generic __exit__ methods (PYI036)

[kpeis695]

Summary

Fixes #25905.

When __exit__ or __aexit__ uses a PEP 695 type parameter bound to BaseException, annotations like type[T] | None and T | None are valid but PYI036 incorrectly flags them:

class Foo:
    def __exit__[T: BaseException](
        self, exc_type: type[T] | None, exc: T | None, tb: TracebackType | None, /
    ) -> bool: ...
# PYI036: The first argument in `__exit__` should be annotated with `object` or `type[BaseException] | None`
# PYI036: The second argument in `__exit__` should be annotated with `object` or `BaseException | None`

The validators is_base_exception_type and the second-arg check only matched type[BaseException] and BaseException literally. The fix passes the function's PEP 695 type params into the validators so type[T] and T are accepted when T is a type parameter whose bound is BaseException.

Type parameters with no bound (or a bound other than BaseException) continue to be flagged as before.

Test Plan

Snapshot tests (cargo nextest run and cargo insta test).

[ntBre]

This also has unrelated changes from #26354. Let's focus on one of these PRs at a time.

[kpeis695]

Closing to reopen from a clean branch (only contains the PYI036 fix, no unrelated commits).