Fix fixture arg detection for bound methods

[cjc0013]

Fixes #6750.

Summary

• keep getfuncargnames(..., cls=...) from removing an implicit receiver when the callable is already a bound method
• preserve existing staticmethod handling and unbound method receiver stripping
• add regression coverage for unbound, bound, and classmethod callables passed with class context

Testing

• python -m compileall -q src\_pytest\compat.py
• manual reproduction for getfuncargnames(A().run_test, name="run_test", cls=A)
• python -m pytest testing/python/fixtures.py::test_getfuncargnames_methods testing/python/fixtures.py::test_getfuncargnames_staticmethod testing/python/fixtures.py::test_getfuncargnames_staticmethod_inherited testing/python/fixtures.py::test_getfuncargnames_staticmethod_partial -q
• python -m pytest testing/python/fixtures.py -q
• uvx ruff check src\_pytest\compat.py testing\python\fixtures.py
• uvx ruff format --check src\_pytest\compat.py testing\python\fixtures.py
• git diff --check

AI Assistance

Prepared with AI assistance. I reviewed the patch and validation output and take responsibility for the submission.

[RonnyPfannschmidt]

Closing as ai slop as the tested behavior and the description contradict each other