A bare token this runner does not own used to be forwarded to every per-file
pytest, so a typo (`--jbs`, or `--help` before #114065's fix) discovered the
whole suite and each file died with "unrecognized arguments" — hours to learn
about a typo. Validate the bare passthrough tokens against pytest's own
argparse parser (installed plugins loaded), and fail once with this runner's
usage (exit 2) before discovery. argparse handles the attached-short-value
(`-rA`), combined-flag (`-xvs`) and `-k expr` forms, so real pytest flags keep
passing through; tokens after a literal `--` are the caller's explicit choice
and are never validated. If pytest's parser cannot be built, the check is
skipped and behaviour is unchanged.
Follow-up to KoNit-K's `-h`/`--help` interception (#114065). Fixes#114059.
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