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/thread_status.rs

@@ -877,10 +877,10 @@ mod tests {
         let OutgoingEnvelope::Broadcast { message } = envelope else {
             panic!("expected broadcast notification");
         };
-        let OutgoingMessage::AppServerNotification(ServerNotification::ThreadStatusChanged(
-            notification,
-        )) = message
-        else {
+        let OutgoingMessage::AppServerNotification(envelope) = message else {
+            panic!("expected thread/status/changed notification");
+        };
+        let ServerNotification::ThreadStatusChanged(notification) = envelope.notification else {
             panic!("expected thread/status/changed notification");
         };
         notification