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/message_processor_tracing_tests.rs

@@ -477,7 +477,7 @@ async fn read_thread_started_notification(
                     continue;
                 };
                 if matches!(
-                    notification,
+                    notification.notification,
                     codex_app_server_protocol::ServerNotification::ThreadStarted(_)
                 ) {
                     return;
@@ -490,7 +490,7 @@ async fn read_thread_started_notification(
                     continue;
                 };
                 if matches!(
-                    notification,
+                    notification.notification,
                     codex_app_server_protocol::ServerNotification::ThreadStarted(_)
                 ) {
                     return;