fix: preserve array items property in union schemas for MCP compatibility

[combiz]

Describe your changes

When converting OpenAPI schemas with Union types containing arrays (e.g., Union[str, List[str]]), the array variant was missing the required 'items' property. This caused validation errors with OpenAI and other LLMs.

Error example:

litellm.BadRequestError: OpenAIException - Error code: 400 - {
  'error': {
    'message': "Invalid schema for function 'stemmed_count_keywords_count_keywords_post_409a8e':
                In context=('properties', 'text'), array schema missing items.",
    'type': 'invalid_request_error',
    'param': 'tools[0].function.parameters',
    'code': 'invalid_function_parameters'
  }
}

The fix introduces simplify_union_schema() which:

• Detects Union types (anyOf/oneOf in OpenAPI)
• Selects the first non-null type from the union
• Preserves ALL properties including 'items' for arrays
• Maintains title, description, and default from parent schema

This ensures compatibility with LLMs that require strict JSON schema validation for function parameters.

Fixes issues with common FastAPI patterns like:

• Union[str, List[str]] - single or multiple values
• Optional[List[T]] - optional arrays
• Union[str, List[Model]] - mixed type unions

Issue ticket number and link (if applicable)

#165

Checklist before requesting a review

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