test: cover status lifecycle fixture states

[Devesh36]

Change and scope

Refs #4.

Adds fixture coverage for the status lifecycle after initialization, binding, and removal. The tests assert the expected summary and installation/binding/unit states, and confirm that inspection leaves the fixture unchanged. Adds unit inventory coverage for a missing systemd unit and unavailable systemctl, including the explicit unknown state.

This PR supplies the issue's requested verification slice; it does not claim the full status outcome is complete.

Verification

• uv run --locked --extra postgres-binary pytest -n 0 --dist loadscope -m 'not container' tests/unit/system/operator/test_inventory.py tests/unit/system/operator/test_units.py — 17 passed.
• uv run --locked --extra postgres-binary ruff check tests/unit/system/operator/test_inventory.py tests/unit/system/operator/test_units.py — passed.
• uv run --locked --extra postgres-binary ruff format --check tests/unit/system/operator/test_inventory.py tests/unit/system/operator/test_units.py — passed.
• make test PYTEST_TARGETS="tests/unit/system/operator/test_inventory.py tests/unit/system/operator/test_units.py" N=0 — wrapper exited with a Bash syntax error before starting pytest; the equivalent focused pytest command above passed.
• No live systemd check was run; these are fixture tests as requested by the issue.

Documentation and delivery claims

None. This PR adds verification evidence only.

Author review

• I reviewed the full diff, including any AI-assisted changes, and can explain the submitted code and its verification.