Register the “benchmark” pytest mark

[musicinmybrain]

Avoids warnings of the form:

PytestUnknownMarkWarning: Unknown pytest.mark.benchmark - is this a
typo?  You can register custom marks to avoid this warning - for
details, see https://docs.pytest.org/en/stable/how-to/mark.html

[gemini-code-assist]

Code Review

This pull request adds a benchmark marker configuration for pytest in pyproject.toml. The reviewer correctly identified that pytest configuration in pyproject.toml must be placed under the [tool.pytest.ini_options] table rather than [tool.pytest] to be recognized, and provided an appropriate code suggestion to fix this issue.

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[ifduyue]

Thank you