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-protocol/src/protocol/common.rs

@@ -1709,6 +1709,25 @@ server_notification_definitions! {
 
 }
 
+/// Server notification envelope sent over app-server transports.
+///
+/// `emitted_at_ms` records when app-server emitted the notification, before it
+/// is fanned out to individual connections.
+#[derive(Serialize, Deserialize, Debug, Clone, TS)]
+#[serde(rename_all = "camelCase")]
+pub struct ServerNotificationEnvelope {
+    #[serde(flatten)]
+    pub notification: ServerNotification,
+    /// Unix timestamp (in milliseconds) when app-server emitted this notification.
+    ///
+    /// Optional so clients can decode notifications from older app-server
+    /// versions. Current app-server versions always populate it.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    #[ts(type = "number")]
+    pub emitted_at_ms: Option<i64>,
+}
+
 client_notification_definitions! {
     Initialized,
 }