implement union type support for streaming actions

[ThyTran1402]

Summary

Extends the @streaming_action.pydantic() decorator to accept union types like Model1 | Model2 or Union[Model1, Model2]) for the stream_type parameter. Previously, only single BaseModel types or dict were accepted, limiting flexibility for streaming actions that yield multiple result types.

Changes

• burr/core/action.py
• burr/integrations/pydantic.py
• tests/integrations/test_streaming_union_types.py

How I tested this

Unit Tests: Created comprehensive test suite with 10 tests that all pass

Notes

Resolved: #607

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.