Set default asyncio fixture loop scope to function

[SahilArchitect]

Closes #924.

Summary

Sets the default value of asyncio_default_fixture_loop_scope to function and removes the deprecated unset
fallback/warning path.

This intentionally only changes the default for async fixture loop scope. It does not change async test loop scope
behavior, custom event_loop fixture deprecations, pytest warning filtering order, or quoted pytest.ini value
parsing.

Changes

• Default unset asyncio_default_fixture_loop_scope to function
• Remove the old fallback to fixture caching scope
• Update docs and changelog wording
• Add regression coverage for the unset/default behavior

Tests

• python -m pytest tests/test_fixture_loop_scopes.py
• python -m pytest tests/test_asyncio_fixture.py tests/modes/test_auto_mode.py tests/modes/test_strict_mode.py

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (74a5557) to head (d70eb97).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1444      +/-   ##
==========================================
+ Coverage   94.50%   94.87%   +0.36%     
==========================================
  Files           2        2              
  Lines         510      507       -3     
  Branches       62       61       -1     
==========================================
- Hits          482      481       -1     
+ Misses         22       21       -1     
+ Partials        6        5       -1     

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