[langgraph] ag-ui langgraph integration does not support concurrent tool call

[happyxgang]

async def _handle_single_event(self, event: Any, state: State) -> AsyncGenerator[str, None]:
    event_type = event.get("event")
    if event_type == LangGraphEventTypes.OnChatModelStream:
        should_emit_messages = event["metadata"].get("emit-messages", True)
        should_emit_tool_calls = event["metadata"].get("emit-tool-calls", True)

        if event["data"]["chunk"].response_metadata.get('finish_reason', None):
            return

        current_stream = self.get_message_in_progress(self.active_run["id"])
        has_current_stream = bool(current_stream and current_stream.get("id"))
        tool_call_data = event["data"]["chunk"].tool_call_chunks[0] if event["data"]["chunk"].tool_call_chunks else None


        is_tool_call_start_event = not has_current_stream and tool_call_data and tool_call_data.get("name")
        is_tool_call_args_event = has_current_stream and current_stream.get("tool_call_id") and tool_call_data and tool_call_data.get("args")
        is_tool_call_end_event = has_current_stream and current_stream.get("tool_call_id") and not tool_call_data

if a tool call is streaming, other tool call will not trigger a tool start event

is this by design or we can improve this