Ignore mock patch arguments when resolving step fixtures

[lin-hongkuan]

Fixes #330.

Summary

unittest.mock.patch() injects mock objects as positional arguments when the decorated function is called. Pytest accounts for that by ignoring those injected arguments while resolving fixtures, but pytest-bdd currently treats every required positional step parameter as a fixture dependency.

That means a patched step like:

@then("the patched function is called")
@patch("pathlib.Path.cwd")
def _(cwd_mock):
    ...

fails before execution with fixture 'cwd_mock' not found.

This updates pytest-bdd's required-argument helper to reuse pytest's num_mock_patch_args() rule, so mock-provided arguments are skipped when resolving step fixtures. Normal fixture dependencies and parser-provided step arguments continue to work unchanged.

Tests

• .\.venv\Scripts\python.exe -m pytest tests/steps/test_common.py::test_step_function_accepts_unittest_mock_patch_argument -q
• .\.venv\Scripts\python.exe -m pytest tests/steps/test_common.py tests/args/parse/test_args.py tests/args/regex/test_args.py tests/args/cfparse/test_args.py -q -> 16 passed
• .\.venv\Scripts\python.exe -m pytest tests/feature/test_steps.py::test_step_hooks tests/feature/test_steps.py::test_step_trace -q -> 2 passed
• .\.venv\Scripts\ruff.exe check src/pytest_bdd/utils.py tests/steps/test_common.py
• .\.venv\Scripts\ruff.exe format --check src/pytest_bdd/utils.py tests/steps/test_common.py
• .\.venv\Scripts\mypy.exe src/pytest_bdd/utils.py -> no type issues; current mypy reports that this version no longer supports the repository's configured python_version = 3.9
• git diff --check