AG UI langgraph python sdk has issue on handling `ToolCallResult` with list output.

I created similar issue on copilotkit issue too: https://github.com/CopilotKit/CopilotKit/issues/2685

<img width="961" height="780" alt="Image" src="https://github.com/user-attachments/assets/15f5a273-5eaf-43a5-b089-59dad20a4bc5" />

The tool result can be a list of string, but currently AGUI langgraph python sdk only supports string.
