Fix: Add validation for asyncio_default_fixture_loop_scope

[pankaj-bind]

This PR addresses an issue where the asyncio_default_fixture_loop_scope configuration option in pytest.ini was not being validated. This could lead to misconfigurations and unexpected behavior without providing any feedback to the user.

This change introduces a validation check in pytest_asyncio/plugin.py to ensure that the provided scope is one of the valid options (function, class, module, package, or session). If an invalid scope is provided, a pytest.UsageError is raised with a descriptive error message.

Additionally, a new test case has been added to tests/test_fixture_loop_scopes.py to verify that an invalid scope configuration correctly raises an error.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.64%. Comparing base (4fbe1e0) to head (b60df08).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1189      +/-   ##
==========================================
- Coverage   88.88%   88.64%   -0.25%     
==========================================
  Files           2        2              
  Lines         405      414       +9     
  Branches       44       46       +2     
==========================================
+ Hits          360      367       +7     
- Misses         35       36       +1     
- Partials       10       11       +1     

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