Limit event_loop_policy parametrization to async tests

[lin-hongkuan]

Summary

• stop parametrized event_loop_policy autouse fixture overrides from applying to synchronous tests that do not request the fixture
• keep explicit synchronous uses of event_loop_policy intact, including direct fixture arguments and usefixtures
• add a regression test covering one async test and one sync test with a parametrized policy fixture

Fixes #796

Tests

• python -m pytest tests\markers\test_event_loop_policy.py -q -p no:twisted
• python -m pytest tests\markers\test_function_scope.py::test_asyncio_mark_respects_parametrized_loop_policies tests\markers\test_function_scope.py::test_event_loop_policy_fixture_override_emits_deprecation_warning tests\markers\test_function_scope.py::test_default_event_loop_policy_fixture_does_not_warn -q -p no:twisted
• python -m pytest tests\markers -q -p no:twisted
• python -m ruff check pytest_asyncio\plugin.py tests\markers\test_event_loop_policy.py
• python -m ruff format --check pytest_asyncio\plugin.py tests\markers\test_event_loop_policy.py
• git diff --check

Note: I used -p no:twisted locally because this environment has an unrelated global pytest-twisted plugin installed, and it interferes with pytester subprocess runs. The pytest-asyncio tests themselves pass with that unrelated plugin disabled.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.30%. Comparing base (c5221cf) to head (3080bc3).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	46.15%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1491      +/-   ##
==========================================
- Coverage   94.50%   93.30%   -1.21%     
==========================================
  Files           2        2              
  Lines         510      523      +13     
  Branches       62       66       +4     
==========================================
+ Hits          482      488       +6     
- Misses         22       25       +3     
- Partials        6       10       +4     

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[tjkuson]

Noting #796 (comment), we should probably be wary of implementing pytest hacks in order to resolve an issue with a deprecated feature.

[tjkuson]

This breaks valid usage for the same reason described here #1454