Adds a conftest.py pytest fixture to explicitly clear the lru_cache for bash
detection, addressing the monkeypatching hazard in a way that doesn't rely
solely on comments. (Note: CI currently runs python -m unittest, but this
keeps local pytest runners safe.)
Also updates the bash check to run the execution probe directly inside the
cached _check_bash_state, exposing the difference between 'no bash found'
and 'bash present but execution failed' directly to the unittest skip reason.
