The #487 fix relies on `save_offline_queue_state` correctly
stamping the session id so the load path's mismatch check has
something to compare against. The existing
`test_offline_queue_round_trip_and_clear` covers
serialization + clear but doesn't pin the session_id stamping
behavior.
Adds `test_offline_queue_stamps_session_id_on_save` which
exercises three cases:
* `save(state, Some("session-A"))` → loaded session_id is
`Some("session-A")`. The stamp made it to disk.
* `save(state, Some("session-B"))` → re-saving replaces the
stamp; loaded session_id is `Some("session-B")`. No stale
ID lingers.
* `save(state, None)` → loaded session_id is `None`. The UI's
load path treats this as legacy-unscoped and refuses to
restore (fail-closed), which is what protects users from
pre-#487 queues leaking into new chats.
Pure additive coverage. The 2 existing offline-queue tests
pass unchanged.
Hunter Bown
0fe05b682a