Had to change the chat_app FastAPI Example to get it to work after copying

[ferrants]

I copied to examples:

python -m pydantic_ai_examples --copy-to examples/

And then tried to run it:

python examples/chat_app.py 

But had it surfacing an error ending with (adding this so someone finds this when searching)

pydantic.errors.PydanticUserError: Dataclass 'ModelRequest' needs a discriminator field for key 'message_kind'

And upon editing the file to remove the MessageTypeAdapter, I saw that it wasn't being used because uvicorn pointed to the installed version, not the current file version. Once I made this change, I was able to run this example.