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/export.rs

@@ -1,6 +1,7 @@
 use crate::ClientNotification;
 use crate::ClientRequest;
 use crate::ServerNotification;
+use crate::ServerNotificationEnvelope;
 use crate::ServerRequest;
 use crate::experimental_api::experimental_fields;
 use crate::export_client_notification_schemas;
@@ -128,6 +129,7 @@ pub fn generate_ts_with_options(
     ServerRequest::export_all_to(out_dir)?;
     export_server_responses(out_dir)?;
     ServerNotification::export_all_to(out_dir)?;
+    ServerNotificationEnvelope::export_all_to(out_dir)?;
 
     if !options.experimental_api {
         filter_experimental_ts(out_dir)?;
@@ -1339,6 +1341,7 @@ where
             strip_v1_client_request_variants_from_json_schema(&mut schema_value);
         } else if file_stem == "ServerNotification" {
             strip_v1_server_notification_variants_from_json_schema(&mut schema_value);
+            add_server_notification_emitted_at_to_json_schema(&mut schema_value)?;
         }
         enforce_numbered_definition_collision_overrides(file_stem, &mut schema_value);
         annotate_schema(&mut schema_value, Some(file_stem));
@@ -1371,6 +1374,25 @@ where
     })
 }
 
+fn add_server_notification_emitted_at_to_json_schema(schema: &mut Value) -> Result<()> {
+    let schema = schema
+        .as_object_mut()
+        .ok_or_else(|| anyhow!("expected ServerNotification schema to be an object"))?;
+    schema.insert(
+        "properties".to_string(),
+        serde_json::json!({
+            "emittedAtMs": {
+                "description": "Unix timestamp (in milliseconds) when app-server emitted this notification.",
+                "format": "int64",
+                "type": "integer"
+            }
+        }),
+    );
+    // Keep this optional in generated client schemas for compatibility with
+    // older app-server versions. New servers still always emit it.
+    Ok(())
+}
+
 fn enforce_numbered_definition_collision_overrides(schema_name: &str, schema: &mut Value) {
     for defs_key in ["definitions", "$defs"] {
         let Some(defs) = schema.get(defs_key).and_then(Value::as_object) else {