Support returning multimodal content such as images from MCP tool calls.

[sandsc]

I noticed that FunctionTool already supports returning multimodal results like ToolOutputImage, but currently invoke_mcp_tool in file util.py converts all content into strings.

[seratch]

Thanks for writing in. If you use_structured_content=True for an MCP server, your MCP server can return structured JSON data instead. We don't plan to support binary data response.