Refactor fixture addfinalizer to utilize ExitStack (#5592)

[giovanamalaia]

Summary
This PR refactors the internal fixture finalizer flow to use contextlib.ExitStack.

What I changed

• Replaced the internal fixture finalizer storage with ExitStack.
• Kept the same teardown behavior:
  • finalizers still run in reverse order (LIFO)
  • all finalizers still run even if one fails
  • teardown errors are still grouped/re-raised as before
• Added a regression test to ensure finalizers from one test do not leak into the next test.

Validation
Ran focused fixture/finalizer tests locally, including addfinalizer and post-finalizer scenarios.

AI Assistance

• GitHub Copilot was used as an assistant during the testing phase, with all changes thoroughly reviewed, tested, and validated locally by the author.

Closes #5592

[RonnyPfannschmidt]

Rejected with prejudice

Putting exitstack thete but then sidestepping it is worse than what we have in place