Some MCP servers (e.g., Pencil) return tool schemas with just
{"type": "object"} without a "properties" field. While this is
valid JSON Schema, some LLM providers reject it with errors like:
Error: Invalid schema for function 'mcp_pencil_get_style_guide_tags':
In context=(), object schema missing properties.
This fix normalizes such schemas by adding an empty properties object
when missing, ensuring compatibility with stricter providers.
