fix: use 'user' as author for synthetic function response event

Address code review feedback: synthetic error responses for orphaned
function calls should consistently use 'user' as the author field,
matching the role='user' already set in the content.

Author: donggyun112

src/google/adk/flows/llm_flows/contents.py

@@ -79,7 +79,6 @@ async def run_async(
 request_processor = _ContentLlmRequestProcessor()
 
 
-
 def _create_synthetic_response_for_orphaned_calls(
     event: Event,
     orphaned_calls: list[types.FunctionCall],
@@ -104,11 +103,12 @@ def _create_synthetic_response_for_orphaned_calls(
 
   return Event(
       invocation_id=event.invocation_id,
-      author=event.author,
+      author='user',
       content=types.Content(role='user', parts=parts),
       branch=event.branch,
   )
 
+
 def _rearrange_events_for_async_function_responses_in_history(
     events: list[Event],
     heal_orphaned_calls: bool = False,