OpenAPI docs generation fails for root return types with Pydantic v2

[martincpt]

Using create_model with __root__ like this:

resp_model = create_model(
    callback.__name__ + "-return",
    __root__=(return_annotation, ...),
)

raises the following error in Pydantic v2:

TypeError: To define root models, use pydantic.RootModel rather than a field called 'root'

Pydantic v2 removed support for defining root models via __root__ in create_model. It now requires using pydantic.RootModel.