app-server: expose loaded thread status via read/list and notifications (#11786)

Motivation
- Today, a newly connected client has no direct way to determine the
current runtime status of threads from read/list responses alone.
- This forces clients to infer state from transient events, which can
lead to stale or inconsistent UI when reconnecting or attaching late.

Changes
- Add `status` to `thread/read` responses.
- Add `statuses` to `thread/list` responses.
- Emit `thread/status/changed` notifications with `threadId` and the new
status.
- Track runtime status for all loaded threads and default unknown
threads to `idle`.
- Update protocol/docs/tests/schema fixtures for the revised API.

Testing
- Validated protocol API changes with automated protocol tests and
regenerated schema/type fixtures.
- Validated app-server behavior with unit and integration test suites,
including status transitions and notifications.

codex-rs/app-server/tests/suite/v2/thread_unarchive.rs

@@ -8,6 +8,7 @@ use codex_app_server_protocol::ThreadArchiveParams;
 use codex_app_server_protocol::ThreadArchiveResponse;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadStartResponse;
+use codex_app_server_protocol::ThreadStatus;
 use codex_app_server_protocol::ThreadUnarchiveParams;
 use codex_app_server_protocol::ThreadUnarchiveResponse;
 use codex_app_server_protocol::ThreadUnarchivedNotification;
@@ -137,6 +138,7 @@ async fn thread_unarchive_moves_rollout_back_into_sessions_directory() -> Result
         unarchived_thread.updated_at > old_timestamp,
         "expected updated_at to be bumped on unarchive"
     );
+    assert_eq!(unarchived_thread.status, ThreadStatus::NotLoaded);
 
     let rollout_path_display = rollout_path.display();
     assert!(