fix(pydantic): allow union of BaseModel types as stream_type

[Ghraven]

Fixes #607

What

Broadens the stream_type parameter type annotation in streaming_action.pydantic() and pydantic_streaming_action() to accept a union of pydantic BaseModel types in addition to a single Type[BaseModel] or Type[dict].

Why

Users who pass a union of models (e.g. MyModel1 | MyModel2) hit a static type error at the call site because the parameter was typed as Union[Type[BaseModel], Type[dict]], which only accepts a single concrete model type. The runtime validation logic in _validate_and_extract_signature_types_streaming() already handles union types correctly — this was a type annotation gap only.

Changes

• burr/core/action.py: stream_type parameter in streaming_action.pydantic() widened to accept object (covers union types, which are not expressible as Type[X]).
• burr/integrations/pydantic.py: Same change to PartialType alias and the validator function signature.
• Docstring updated to mention union support.

Testing

No behaviour change — this is a type annotation fix only. Existing tests continue to pass. Users can now write:

union = MyModel1 | MyModel2

@streaming_action.pydantic(
    reads=[...],
    writes=[...],
    state_input_type=...,
    state_output_type=...,
    stream_type=union,
)

without a type error.