fix(core): resolve $ref to $defs defined in nested property schemas

[Krishnachaitanyakc]

Summary

Fixes #7129

When a JSON Schema property defines its own $defs (e.g., an MCP tool schema where a nested parameter object contains enum definitions), schema_to_pydantic_model fails with ReferenceNotFoundError because _json_schema_to_model only processes $defs at the root schema level.

This is triggered when using mcp_server_tools() with MCP servers whose tool input schemas contain nested $defs -- for example, mcp-server-everything-search defines a WindowsSortOption enum inside the windows_params property's $defs.

Error before fix:

autogen_core.utils._json_to_pydantic.ReferenceNotFoundError: Reference `WindowsSortOption` not found in cache. Available: []

Changes

• In _json_schema_to_model, detect and merge nested $defs into the root schema's $defs so _resolve_ref can locate them
• Cache object-type nested definitions as models in _model_cache
• Resolve non-model definitions (enums, primitives) inline when encountered via $ref in properties, avoiding the incorrect creation of empty BaseModels for non-object types
• Added three new tests covering: nested enum $defs (exact reproduction of When using mcp_server_tools, an error occurs, but it works normally with McpWorkbench. #7129), nested object $defs, and multiple nested $defs across different properties

Test plan

• test_nested_defs_enum_ref -- reproduces the exact schema from issue When using mcp_server_tools, an error occurs, but it works normally with McpWorkbench. #7129
• test_nested_defs_object_ref -- nested $defs with object-type definitions
• test_multiple_nested_defs -- multiple properties each with their own $defs
• Verified all existing tests in test_json_to_pydantic.py continue to pass (top-level $defs, self-referencing, allOf merging, etc.)

[Krishnachaitanyakc]

@ekzhu can you please review these changes?