fix(xl-ai): preserve error messages in ClientSideTransport stream

The AI SDK changed toUIMessageStream()'s default onError to return a
generic "An error occurred." instead of the real message (to prevent
leaking server details). Since ClientSideTransport runs entirely
client-side, explicitly pass onError to surface the original error.

Author: nperez0111

packages/xl-ai/src/streamTool/vercelAiSdk/clientside/ClientSideTransport.ts

@@ -95,7 +95,10 @@ export class ClientSideTransport<
       // activeTools: ["applyDocumentOperations"],
     });
 
-    return ret.toUIMessageStream();
+    return ret.toUIMessageStream({
+      onError: (error) =>
+        error instanceof Error ? error.message : String(error),
+    });
   }
 
   async sendMessages({