Fix: FastAPI /docs OpenAPI schema generation for PydanticObjectId

[nuclearcat]

Add custom JSON schema generator to handle Beanie's PydanticObjectId validator which was causing schema generation to fail. The custom generator provides a string schema representation for ObjectId fields, allowing the OpenAPI documentation to be generated successfully.

[nuclearcat]

Tried:

1. Custom GenerateJsonSchema class: Created a custom schema generator to intercept and handle PydanticObjectId schemas - did not work as the generator wasn't called at the right level.

2. Monkey-patching JSON schema method: Replaced PydanticObjectId.__get_pydantic_json_schema__ with customimplementation - did not work because Beanie's existing method was still calling the problematic handler.

3. Monkey-patching both core and JSON schema methods: Replaced both __get_pydantic_core_schema__ and __get_pydantic_json_schema__ with string-based implementations - still not working.

We might get rid of fastapi-users to solve this issue

[JenySadadia]

Hello, out of curiosiry, I checked /docs endpoint on my local setup. It's working.