Apply loop factories when asyncio is marked via parametrize

[gyanu2507]

pytest_generate_tests only looked at the function's own asyncio marker, so this never picked up loop factories:

@pytest.mark.parametrize(
    "backend",
    [pytest.param("asyncio", marks=pytest.mark.asyncio)],
)
async def test_async(backend):
    ...

Also convert those functions during collection so strict mode still owns them.

Fixes #1463.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 45.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.62%. Comparing base (40342fa) to head (8aca36b).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	45.00%	4 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1551      +/-   ##
==========================================
- Coverage   94.50%   92.62%   -1.89%     
==========================================
  Files           2        2              
  Lines         510      529      +19     
  Branches       62       71       +9     
==========================================
+ Hits          482      490       +8     
- Misses         22       26       +4     
- Partials        6       13       +7     

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