Use issue_config_time_warning for the unset-loop-scope deprecation

[mathewOracle]

pytest_configure warns via a plain warnings.warn() when asyncio_default_fixture_loop_scope is unset. Per pytest's own docs on Config.issue_config_time_warning, warnings raised this way during pytest_configure can't be captured the way per-test warnings are -- pytest has no way to install a hookwrapper around pytest_configure -- so the warning is silently dropped: it never reaches the warnings summary, and doesn't respect -W/filterwarnings.

$ pytest  # asyncio_default_fixture_loop_scope left unset
2 passed in 0.01s   # no warning shown anywhere, including with -W default

Depending on the caller's global warning filter state it can also raise an uncaught PytestDeprecationWarning straight out of pytest_configure, crashing the run outright (reproducible via pytester's in-process runpytest(), which shares the outer process's filters).

Config.issue_config_time_warning is pytest's documented mechanism for exactly this case, and is what pytest's own core plugins use for their config-time deprecations (e.g. _pytest/pastebin.py). Switching to it makes the warning show up in the warnings summary and respect the caller's filters, with no change to the message or when it fires.

Verified live against the exact repro from the issue: the warning now appears in a plain pytest run with no flags needed. Added a regression test that fails against the previous code (either silently, or via INTERNALERROR depending on inherited filters) and passes with this change, plus a sanity test that no warning fires when the option is configured. Full test suite shows identical pre-existing failures/errors before and after this change (unrelated compatibility gaps with the installed pytest version); no new failures introduced. ruff check clean.

Closes #1142

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.29%. Comparing base (dcd61c2) to head (fe20426).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1557      +/-   ##
==========================================
+ Coverage   94.50%   95.29%   +0.78%     
==========================================
  Files           2        2              
  Lines         510      510              
  Branches       62       62              
==========================================
+ Hits          482      486       +4     
+ Misses         22       20       -2     
+ Partials        6        4       -2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.