[unattended ai] fix(fixtures): clear stale finalizers when fixture setup fails before caching

[okxint]

Fixes #14775.

Problem

When -Werror turns a PytestRemovedIn10Warning into an error during fixture setup, FixtureDef.execute() exits without setting cached_result. The pytest_fixture_post_finalizer lambda that was just registered in self._finalizers stays there permanently — finish() is a no-op when cached_result is None (it guards with "already finished").

The next test that uses the same FixtureDef calls execute() again and hits:

assert not self._finalizers
AssertionError

This internal AssertionError masks the real error (the deprecation warning), making the second test's failure confusing.

Root cause

execute() for test_1:
  self._finalizers.append(pytest_fixture_post_finalizer)  # line 1281
  ihook.pytest_fixture_setup(...)  # RAISES — warning-as-error
  # cached_result never set; _finalizers = [<stale>]

execute() for test_2:
  assert not self._finalizers  # BOOM — stale entry from test_1

Fix

Clear self._finalizers at the top of the "fresh execution" path, before any new finalizers are registered. This is safe:

• No previous setup ran → list is already empty, clear() is a no-op.
• Previous setup succeeded → finish() already cleared _finalizers.
• Previous setup failed → stale entries are removed; fresh execution gets a clean slate.

Result

Both test_1 and test_2 now report PytestRemovedIn10Warning (the real error) instead of test_2 reporting an internal AssertionError.

Test

Added test_class_scoped_instance_method_werror_multiple_tests in testing/deprecated_test.py that reproduces the exact scenario from the issue and asserts no AssertionError appears.