fix: support Union types for stream_type parameter

[leobin001]

Fixes #607

Summary

This PR allows stream_type parameter in @streaming_action.pydantic to accept Union types like MyModel1 | MyModel2.

Problem

Previously, the type hint was:

stream_type: Union[Type[BaseModel], Type[dict]]

This caused type checkers to complain when users tried to pass Union types like:

union = MyModel1 | MyModel2

@streaming_action.pydantic(
    ...
    stream_type=union,  # Type error!
)

Solution

Changed the type hints to typing.Any to support Union combinations of BaseModel types while maintaining backward compatibility.

Changes

• Updated PartialType in burr/integrations/pydantic.py to typing.Any
• Updated stream_type parameter in streaming_action.pydantic() to Any
• Updated _validate_and_extract_signature_types_streaming() signature

Testing

The change is backward compatible - all existing code using Type[BaseModel] or Type[dict] will continue to work, and now Union types are also supported.

[leobin001]

Closing this PR as I discovered #612 was submitted earlier (Dec 30, 2025) with a better solution that includes proper type hints and test cases.

I apologize for the duplicate - I should have checked more carefully for existing PRs before starting work.

Thanks to @majiayu000 for the comprehensive fix!