fix(benchmarks): skip benchmark tests without pytest-codspeed

[cbueth]

This PR fixes the benchmark suite failing when pytest-codspeed is not installed, e.g. when conda-forge runs pytest tests/ against the sdist.

tests/benchmarks/ was added in 0.3.3. Its tests request the benchmark fixture (provided only by pytest-codspeed) and use @pytest.mark.benchmark, a marker the plugin registers. The old skip logic in tests/benchmarks/conftest.py only ran when the plugin was importable and silently did nothing otherwise. Without the plugin, pytest tests/ aborted with PytestUnknownMarkWarning for the unregistered benchmark mark (escalated to an error by filterwarnings = ["error"]) and fixture 'benchmark' not found.

register benchmark / codspeed_benchmark markers in pytest_configure when not already present.
identify benchmark tests by the benchmark fixture in item.fixturenames (same check the plugin uses) instead of importing the plugin, so skipping works with or without pytest-codspeed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (da07a73) to head (fd93231).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #63   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           29        29           
  Lines          866       866           
  Branches       150       150           
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  Hits           866       866           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 28 untouched benchmarks

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_{Comparing fix/benchmark-conftest-codspeed-optional (fd93231) with main (3eee1cc)¹}

Footnotes

1. No successful run was found on main (da07a73) during the generation of this report, so 3eee1cc was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩