fix: ignore missing user systemd unit in timeout check

[asdfafadfadsfa]

Summary

• Ignore the user-manager default TimeoutStopUSec when the queried service has LoadState=not-found.
• Fall through to the system-level unit, avoiding a false stale-unit warning for system services.
• Add a regression test covering a missing user unit followed by a 240-second system unit.

Test plan

• scripts/run_tests.sh tests/gateway/test_shutdown_forensics.py -v --tb=short (11 passed)

[alt-glitch]

This was generated by AI during triage.

Duplicate of #34734: both gate the systemd timeout check on LoadState before trusting a manager's synthesized default. #34734 handles all non-loaded states, while this is the narrower not-found variant.

[asdfafadfadsfa]

Closing as a duplicate of #34734. Both changes address the same systemctl --user show false-positive: synthesized timeout defaults must not be trusted unless LoadState=loaded. #34734 handles all non-loaded states, whereas this PR covered only the narrower missing/not-found case.