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/src/thread_state.rs

@@ -125,16 +125,13 @@ impl ThreadStateManager {
                 self.thread_ids_by_connection
                     .remove(&subscription_state.connection_id);
             }
-            if let Some(thread_state) = self.thread_states.get(&thread_id) {
-                thread_state
-                    .lock()
-                    .await
-                    .remove_connection(subscription_state.connection_id);
-            }
         }
 
         if let Some(thread_state) = self.thread_states.get(&thread_id) {
             let mut thread_state = thread_state.lock().await;
+            if !connection_still_subscribed_to_thread {
+                thread_state.remove_connection(subscription_state.connection_id);
+            }
             if thread_state.subscribed_connection_ids().is_empty() {
                 thread_state.clear_listener();
             }