fix(pydantic): accept union type objects for stream_type hints

[majiayu000]

Summary

• treat union type objects as valid stream_type values for pydantic streaming actions
• keep runtime behavior unchanged while relaxing type hints for union-type streams

Testing

• not run (type-hint only change)

[skrawcz]

doing Any for typing checking will of course work and is cheating...

I think we shouldn't have a typing.TYPE_CHECKING part here, and instead I think the fix is line 74 + 75.

note I had to do this stream_type: Union[Type["BaseModel"], Type[dict], type, "types.UnionType"], for my pyright in the IDE to not complain -- i.e. the quotes part.

[majiayu000]

Thanks for the review! I've updated the code according to your suggestion:

• Removed the TYPE_CHECKING + Any approach
• Added "types.UnionType" (with quotes) to the Union type hints

The type definitions are now:

# burr/integrations/pydantic.py
PartialType = Union[Type[pydantic.BaseModel], Type[dict], type, "types.UnionType"]

# burr/core/action.py  
stream_type: Union[Type["BaseModel"], Type[dict], type, "types.UnionType"]

This should satisfy pyright while properly accepting union type objects like str | int for the stream_type parameter.