Killing the last session shuts the server down, and tmux >= 3.7 reports
that to the still-connected client with a phrasing `is_no_server_error`
did not match. `list_sessions` then returned Err, and `run_liveness_pass`
logs a warning and returns — so the pass reaped nothing.
Silent by construction: nothing asserted the phrasing set, and the only
symptom was an integration test that looked flaky. Both now covered.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
