fix(result): support Pydantic model_rebuild for RunResultStreaming

[OiPunk]

Summary

Fixes Pydantic undefined-annotation errors when RunResultStreaming is used as a field type in a Pydantic model and model_rebuild() is called.

What changed

• Added RunResultBase.__get_pydantic_core_schema__ so run-result objects are treated as runtime instance types during Pydantic schema generation.
• Added a regression test for the reported pattern (RunResultStreaming | None + ConfigDict(arbitrary_types_allowed=True) + model_rebuild()).

Why this works

Pydantic was recursively traversing internal dataclass annotations from RunResultStreaming, which triggers unresolved forward references like Agent in nested runtime types. Treating run results as instance types avoids recursive traversal while preserving normal runtime usage.

Validation

• uv run ruff check src/agents/result.py tests/test_result_cast.py
• uv run pytest tests/test_result_cast.py -q
• uv run pytest tests/test_source_compat_constructors.py -q
• Manual repro script from the issue now succeeds (MyModel.model_rebuild() no longer raises).

Fixes #2127