Proxy plugin fails to resolve multipart file fields

[touale]

When using the proxy plugin to dynamically generate FastAPI endpoints from an OpenAPI schema, two issues occur:

• 1. $ref schema resolution may fail with error:
     schema ‘’ not found in components
• 2. multipart/form-data file fields are not handled correctly:
     even when resolved to UploadFile, Swagger UI still shows them as string and does not render file upload input

[touale]

Fixed it in #102