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-protocol/src/protocol/common.rs

@@ -774,6 +774,7 @@ server_notification_definitions! {
     /// NEW NOTIFICATIONS
     Error => "error" (v2::ErrorNotification),
     ThreadStarted => "thread/started" (v2::ThreadStartedNotification),
+    ThreadStatusChanged => "thread/status/changed" (v2::ThreadStatusChangedNotification),
     ThreadArchived => "thread/archived" (v2::ThreadArchivedNotification),
     ThreadUnarchived => "thread/unarchived" (v2::ThreadUnarchivedNotification),
     ThreadNameUpdated => "thread/name/updated" (v2::ThreadNameUpdatedNotification),
@@ -1334,6 +1335,28 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_thread_status_changed_notification() -> Result<()> {
+        let notification =
+            ServerNotification::ThreadStatusChanged(v2::ThreadStatusChangedNotification {
+                thread_id: "thr_123".to_string(),
+                status: v2::ThreadStatus::Idle,
+            });
+        assert_eq!(
+            json!({
+                "method": "thread/status/changed",
+                "params": {
+                    "threadId": "thr_123",
+                    "status": {
+                        "type": "idle"
+                    },
+                }
+            }),
+            serde_json::to_value(&notification)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn mock_experimental_method_is_marked_experimental() {
         let request = ClientRequest::MockExperimentalMethod {