Add tests for enforce_rate_limit()

[SemTiOne]

rate_limiter.py has 17 tests but none call enforce_rate_limit() itself, only its building blocks (check_cooldown, check_daily_cap, etc.). This
function had a real ordering bug fixed in an earlier audit, it deserves direct coverage.

Untested:

• rate_limit.enabled=False → returns immediately
• force=True → bypasses both checks
• Cooldown active → sys.exit(1)
• Daily cap reached → sys.exit(1)
• Both checks pass → returns normally

Use pytest.raises(SystemExit) for the exit paths. Monkeypatch rate_limiter.USAGE_PATH to a tmp_path file (see existing tests in the same file for the pattern).