Include class docstrings as description in OpenAPI schema

[renan-r-santos]

Class docstrings were not showing up as "description" in the OpenAPI schema. This is because Pydantic v2 delegates schema generation entirely to __get_pydantic_json_schema__, which we override, and we weren't injecting cls.__doc__ into the result.

[drhagen]

I moved the automatic __doc__ implementation to the mixins, because that felt like a more appropriate place to do this. Let me know whether or not that meets your need.

[renan-r-santos]

I moved the automatic __doc__ implementation to the mixins, because that felt like a more appropriate place to do this. Let me know whether or not that meets your need.

That's way better, thanks! And I confirmed it works fine too.