When a prompt function returned `mcp.types.PromptMessage` objects directly
and was executed as a task, the result serialization failed with
"'PromptMessage' object has no attribute 'to_mcp'".
The task result converter was calling `.to_mcp()` on what it thought was a
FastMCP wrapper type, but the import actually pulls in `mcp.types.PromptMessage`
directly, which is already the final MCP type. Removed the unnecessary
conversion.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
