test(service): tighten systemctl status parsing

[vernonstinebaker]

Summary

• tighten systemctl availability detection so generic "not found" text does not misclassify missing service units as a missing systemctl binary
• add focused regression coverage for missing-command detection and systemd user-session detection helpers
• keep the change isolated to service status parsing so API behavior remains stable while error classification becomes more accurate

Validation

• zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• npm --prefix ui ci --no-audit --no-fund
• npm --prefix ui run build
• bash tests/test_e2e.sh
• zig fmt --check src/
• git diff --check

Notes

• this is intentionally a boundary-only slice: it protects the helper logic that decides whether to report "systemctl unavailable" versus a normal command failure
• a typical missing-unit response like Unit nullhub.service could not be found. should no longer be treated as if the systemctl executable itself is absent