Fix PytestWarning: usefixtures() without arguments when scenario has no fixture args

[Copilot]

pytest 9.x added a PytestWarning when usefixtures() is called with no arguments. This surfaced in pytest-bdd whenever a scenario function had no fixture arguments (e.g. test functions auto-generated by scenarios()), because pytest.mark.usefixtures(*func_args) was called unconditionally even with an empty func_args.

Changes

• scenario.py: Guard the usefixtures mark application with if func_args: so it is only applied when there are actual fixture names to pass.
• tests/feature/test_scenario.py: Add regression test test_no_usefixtures_warning_when_no_args that runs pytest with -W error::pytest.PytestWarning to confirm no spurious warning is emitted when using scenarios() with no-argument test functions.

# Before: always applied, even with func_args=[]
scenario_wrapper = pytest.mark.usefixtures(*func_args)(scenario_wrapper)

# After: only applied when there are fixture names
if func_args:
    scenario_wrapper = pytest.mark.usefixtures(*func_args)(scenario_wrapper)