AttributeError: root on self.__pydantic_serializer__.to_python after upgrade Fastapi to 0.103.1

[liguifan]

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Issue Content

After update fastapi to 0.103.1, the RootModel cannot be serialized anymore. Here is my SerializableMessage class:

class SerializableMessage(RootModel):
    root: Message

    class Config:
        arbitrary_types_allowed = True  # for Message

the below server_debug_summary contains SerializableMessage:

File "N:\Pycharm\xxxx\src\app\web\views\debug_view.py", line 54, in _compute_debug_summary
debug_view_data.update(jsonable_encoder(server_debug_summary))
File "N:\Pycharm\xxxx\venv\lib\site-packages\fastapi\encoders.py", line 131, in jsonable_encoder
obj_dict = _model_dump(
File "N:\Pycharm\xxxx\venv\lib\site-packages\fastapi_compat.py", line 171, in _model_dump
return model.model_dump(mode=mode, **kwargs)
File "N:\Pycharm\xxxx\venv\lib\site-packages\pydantic\main.py", line 308, in model_dump
return self.pydantic_serializer.to_python(
AttributeError: root