fix(gateway): ignore unloaded systemd units in forensics

[Pluviobyte]

Summary

• query systemd LoadState together with TimeoutStopUSec before trusting shutdown timing data
• skip missing, non-loaded, or incomplete unit records so nonexistent user units do not produce false mismatch diagnostics
• add regression coverage for loaded, not-found fallback, non-loaded states, and missing LoadState outputs

Fixes #36755.

Validation

• .venv/bin/python -m pytest tests/gateway/test_shutdown_forensics.py::TestCheckSystemdTimingAlignment -q
• .venv/bin/python -m pytest tests/gateway/test_shutdown_forensics.py -q -k "not test_spawns_subprocess_and_writes_output"

Note: the full test file has one local macOS environment failure in test_spawns_subprocess_and_writes_output because the diagnostic spawn returns None here; the new systemd timing coverage passes.

[teknium1]

Thanks for the focused regression fix. The premise remains present on current main: gateway/shutdown_forensics.py:367-389 queries systemctl --user first and trusts TimeoutStopUSec without checking the unit's load state, while gateway/run.py:6733-6745 consumes that result for the startup warning.

The PR's LoadState=loaded gate addresses the documented not-found default-value path and correctly permits the subsequent system-manager query. Its tests cover the loaded case, user-to-system fallback, non-loaded replies, and omitted LoadState. GitHub currently reports the PR as mergeable; the narrow change should be directly salvageable.

Automated hermes-sweeper review.