chore(logging): log the problematic event for #12122 (#14092)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/a2a-server/src/agent/task.ts

@@ -662,7 +662,7 @@ export class Task {
           errorMessage,
         );
 
-        let errMessage = 'Unknown error from LLM stream';
+        let errMessage = `Unknown error from LLM stream: ${JSON.stringify(event)}`;
         if (errorEvent.value) {
           errMessage = parseAndFormatApiError(errorEvent.value);
         }