Feat: add model reroute notification (#12001)

### Summary
Builiding off
5c75aa7b89 (diff-058ae8f109a8b84b4b79bbfa45f522c2233b9d9e139696044ae374d50b6196e0),
we have created a `model/rerouted` notification that captures the event
so that consumers can render as expected. Keep the `EventMsg::Warning`
path in core so that this does not affect TUI rendering.

`model/rerouted` is meant to be generic to account for future usage
including capacity planning etc.

codex-rs/app-server/src/outgoing_message.rs

@@ -399,6 +399,8 @@ mod tests {
     use codex_app_server_protocol::AuthMode;
     use codex_app_server_protocol::ConfigWarningNotification;
     use codex_app_server_protocol::LoginChatGptCompleteNotification;
+    use codex_app_server_protocol::ModelRerouteReason;
+    use codex_app_server_protocol::ModelReroutedNotification;
     use codex_app_server_protocol::RateLimitSnapshot;
     use codex_app_server_protocol::RateLimitWindow;
     use codex_protocol::ThreadId;
@@ -546,6 +548,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn verify_model_rerouted_notification_serialization() {
+        let notification = ServerNotification::ModelRerouted(ModelReroutedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            from_model: "gpt-5.3-codex".to_string(),
+            to_model: "gpt-5.2".to_string(),
+            reason: ModelRerouteReason::HighRiskCyberActivity,
+        });
+
+        let jsonrpc_notification = OutgoingMessage::AppServerNotification(notification);
+        assert_eq!(
+            json!({
+                "method": "model/rerouted",
+                "params": {
+                    "threadId": "thread-1",
+                    "turnId": "turn-1",
+                    "fromModel": "gpt-5.3-codex",
+                    "toModel": "gpt-5.2",
+                    "reason": "highRiskCyberActivity",
+                },
+            }),
+            serde_json::to_value(jsonrpc_notification)
+                .expect("ensure the notification serializes correctly"),
+            "ensure the notification serializes correctly"
+        );
+    }
+
     #[tokio::test]
     async fn send_response_routes_to_target_connection() {
         let (tx, mut rx) = mpsc::channel::<OutgoingEnvelope>(4);