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 88.67%. Comparing base (4fbe1e0) to head (4bd8a3b).
⚠️ Report is 10 commits behind head on main.

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%   88.67%   -0.22%     
==========================================
  Files           2        2              
  Lines         405      415      +10     
  Branches       44       46       +2     
==========================================
+ Hits          360      368       +8     
- Misses         35       36       +1     
- Partials       10       11       +1     

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[seifertm]

This is a great idea! . It will give better error messages in case the user has a typo in their configuration. The code and test look solid. The only thing that's left is notifying the users about this addition in the changelog.

To add a changelog entry, you can simply add a file (aka news fragment) to changelog.d/ stating that incorrect configuration values for default_fixture_loop_scope and default_test_loop_scope now give a better error message. The news fragment should point to the underlying issue or, in this case, the PR and should have the category added. That means the complete file path for the news item is changelog.d/1189.added.rst.

I'd also appreciate if you could squash the commits in this PR (just this once), so we don't have the fixup commits from pre-commit CI in the Git history.