Timestamp app-server notifications at emission (#32905)

## What changed

- Add an optional `emittedAtMs` field to server notification envelopes and expose the envelope in the generated JSON and TypeScript schemas.
- Populate the Unix timestamp when app-server emits a notification, before transport routing, so every connection in a fan-out receives the same value.
- Keep the field optional for clients decoding notifications from older app-server versions.

## Testing

- Verify notification serialization includes `emittedAtMs`.
- Verify targeted fan-out reuses one timestamp across connections.

GitOrigin-RevId: b379cb11e8e91c558bcd3d46ce459311a6e81e54

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

@@ -667,7 +667,8 @@ async fn start_uninitialized(args: InProcessStartArgs) -> IoResult<InProcessClie
                                     .await;
                             }
                         }
-                        OutgoingMessage::AppServerNotification(notification) => {
+                        OutgoingMessage::AppServerNotification(envelope) => {
+                            let notification = envelope.notification;
                             if server_notification_requires_delivery(&notification) {
                                 if event_tx
                                     .send(InProcessServerEvent::ServerNotification(notification))