fix: default asyncio_default_fixture_loop_scope to "function"

[sridhar-3009]

Summary

Closes #924.

The asyncio_default_fixture_loop_scope ini option was left None by default, which triggered a PytestDeprecationWarning on every test run and caused async fixtures to fall back to the old fixture-caching scope behaviour.

This PR completes the deprecation by:

1. Setting the default to "function" — the value the deprecation warning was telling users to set explicitly.
2. Removing _DEFAULT_FIXTURE_LOOP_SCOPE_UNSET — the warning constant is now unreachable since the default is no longer falsy.
3. Removing the if not default_fixture_loop_scope: warnings.warn(...) block in pytest_configure.

Before

# No ini setting needed, but you got a warning every run:
# PytestDeprecationWarning: The configuration option
#   "asyncio_default_fixture_loop_scope" is unset. ...

After

# No ini setting needed, no warning.
# Fixtures default to "function" scope.

Follow-up

Tests that explicitly set asyncio_default_fixture_loop_scope = function in pytester configs are now setting the default redundantly. Cleaning those up is a separate, mechanical follow-up.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (6e14cd2) to head (9733864).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1462      +/-   ##
==========================================
+ 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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[golikovichev]

Quick heads-up: there are two other open PRs against the same issue (#924):

• Set default asyncio fixture loop scope to function #1444 by @SahilArchitect - set default to "function" and adds a regression test that asserts the warning is gone. Already has maintainer review feedback applied. Most recent activity was 2026-05-15, looks close to landing.
• fix: Finish deprecation of unset asyncio_default_fixture_loop_scope #1381 by @JiwaniZakir - older single-line default change, no test, no human review yet.

This PR is closest in shape to #1381 (drop the warning block, set the default), and the diff overlaps almost entirely with the plugin.py change in #1444. To keep things tidy for the maintainers, would you be open to either:

1. Closing this PR in favour of Set default asyncio fixture loop scope to function #1444 and dropping a review there (an "approves" or a concrete comment on the regression test would probably help that one land faster), or
2. Rebasing here on top of Set default asyncio fixture loop scope to function #1444 once it merges, if there's something left to do.

If you spotted a behaviour gap that #1444 doesn't cover, that's worth flagging on the issue thread so all three PRs converge on the same target rather than triaging the same default change three times.

[sridhar-3009]

Closing in favour of #1444 which is further along and has maintainer feedback applied. Will follow that PR instead.