fix: preserve tool input enum schemas

[zxyasfas]

Summary

• Preserve JSON Schema enum values when converting MCP tool input schemas into Pydantic model fields.
• Use Literal types for primitive enum values so generated OpenAPI schemas expose allowed values and validation stays aligned with the MCP schema.
• Keep enum schema metadata when aliased parameter fields are generated.

Fixes #245

Testing

• uv run pytest

Changelog Entry

Fixed

• Preserve enum constraints from MCP tool input schemas in generated OpenAPI request schemas.

Additional Information

• No new dependencies.