"\x00" in script_path raises TypeError when a caller passes a non-str
(e.g. a pathlib.Path, which is not iterable) — the guard itself would
crash the scheduler. All current call sites pass plain str, but the
guard must be crash-proof: str() first, then check. Adds a regression
test running a real script through _run_job_script with a Path argument,
which fails with TypeError on the pre-fix guard.
795e035f60