Fix return type of send_internal_notification

Match the underlying manager method's Union return type since the async
branch returns AsyncTaskResultSummary.

Author: mvdbeek

lib/galaxy/webapps/galaxy/services/notifications.py

@@ -54,7 +54,7 @@ def notifications_enabled(self) -> bool:
 
     def send_internal_notification(
         self, request: NotificationCreateRequest, force_sync: bool = False
-    ) -> NotificationCreatedResponse:
+    ) -> Union[NotificationCreatedResponse, AsyncTaskResultSummary]:
         """Send a system-emitted notification on behalf of internal callers (e.g. share flows).
 
         Unlike :meth:`send_notification`, this skips admin/permission checks because the