Move the large embedded `_BEHAVE_PARALLEL_CLI_SOURCE` string constant out of
noxfile.py and into a standalone `scripts/run_behave_parallel.py` module.
The `_install_behave_parallel()` helper now reads the script from disk via
`Path(__file__).parent / 'scripts' / 'run_behave_parallel.py'` instead of
embedding the source as a raw string literal. This allows ruff to lint and
type-check the runner independently, and makes noxfile.py significantly
shorter and easier to read.
No functional changes: the installed `behave-parallel` entry point is
identical to the previous embedded version. Parallel and sequential modes,
coverage integration, and the multiprocessing fork model are all preserved.
Fixed two SIM105 lint violations in the extracted script (replaced
try/except/pass with contextlib.suppress).
ISSUES CLOSED: #1538
