Describe your changes

Added circular reference detection to prevent RecursionError when processing recursive Pydantic models. The solution uses visited_paths tracking to detect circular references and returns placeholder schemas to break infinite recursion cycles.

Issue ticket number and link (if applicable)

Fixes #238

Screenshots of the feature / bugfix

N/A - This is a backend fix for schema resolution

Checklist before requesting a review

• Added relevant tests
• Run ruff & mypy
• All tests pass

Technical Details

Problem: RecursionError when processing recursive Pydantic models with circular references

Solution:

• Add visited_paths tracking to detect circular references
• Return placeholder schema when circular reference detected
• Maintain backward compatibility

How it works:
When a circular reference is detected:

1. First occurrence: Normal schema resolution with all properties
2. Subsequent occurrences: Placeholder schema with {"type": "object", "description": "Circular reference detected", "properties": {}}

Impact & Limitations:
✅ Preserved functionality:

• All non-circular properties remain fully accessible
• First-level schema information is complete
• API documentation remains clear and usable

⚠️ Limitations:

• Deep nested structures beyond the circular reference point show placeholder
• Infinite depth nesting is not supported (by design)

Changes:

• Modified resolve_schema_references() in fastapi_mcp/openapi/utils.py
• Added comprehensive test suite for circular reference scenarios
• All existing tests pass