fix: preserve empty JSON schemas for tool parameters

[celia-oai]

Why

MCP and dynamic tool schemas can use an empty JSON Schema object ({}) to mean an unconstrained value. The tool schema sanitizer previously inferred string when no schema keywords were present, which narrowed {} into { "type": "string" }.

That misrepresents schemas for tools such as Notion page creation, where fields like body.parent may be object-shaped payloads even when the upstream schema leaves the field unconstrained. Advertising those fields as strings gives the model and any downstream schema validation the wrong contract.

What changed

• Preserve exact empty schema objects in sanitize_json_schema instead of inferring string.
• Add an explicit JsonSchema::any constructor for the internal unconstrained schema representation.
• Update JSON schema tests so top-level {} and nested property schemas such as body.parent: {} remain unconstrained.