avoid concat tool call delta id

Author: DeoLeung

src/agents/models/chatcmpl_stream_handler.py

@@ -276,7 +276,7 @@ async def handle_stream(
  state.function_calls[tc_delta.index].name += (
  tc_function.name if tc_function else ""
  ) or ""
- state.function_calls[tc_delta.index].call_id += tc_delta.id or ""
+ state.function_calls[tc_delta.index].call_id = tc_delta.id or ""
 
  if state.reasoning_content_index_and_output:
  yield ResponseReasoningSummaryPartDoneEvent(