Python: fix Foundry handoff argument serialization

[he-yufeng]

Summary

• serialize dataclass arguments in Foundry hosting function-call output streams
• keep existing string arguments unchanged and serialize framework objects through to_dict() when available
• add a streaming regression test for handoff-like request arguments

Fixes #5858

To verify

• uv run --package agent-framework-foundry-hosting pytest packages\foundry_hosting\tests\test_responses.py -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run --package agent-framework-foundry-hosting pytest packages\foundry_hosting\tests\test_responses.py::TestStreaming -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run ruff check packages\foundry_hosting\agent_framework_foundry_hosting\_responses.py packages\foundry_hosting\tests\test_responses.py
• uv run ruff format --check packages\foundry_hosting\agent_framework_foundry_hosting\_responses.py packages\foundry_hosting\tests\test_responses.py
• uv run python -m py_compile packages\foundry_hosting\agent_framework_foundry_hosting\_responses.py packages\foundry_hosting\tests\test_responses.py
• uv run mypy --config-file packages\foundry_hosting\pyproject.toml packages\foundry_hosting\agent_framework_foundry_hosting
• git diff --check

[he-yufeng]

Rebased onto current main and resolved the overlap with the newer Foundry Hosting response changes.

Local validation:

uv run --package agent-framework-foundry-hosting pytest packages\foundry_hosting\tests\test_responses.py -q --basetemp .tmp\pytest -p no:cacheprovider
uv run ruff check packages\foundry_hosting\agent_framework_foundry_hosting\_responses.py packages\foundry_hosting\tests\test_responses.py
uv run ruff format --check packages\foundry_hosting\agent_framework_foundry_hosting\_responses.py packages\foundry_hosting\tests\test_responses.py
uv run python -m py_compile packages\foundry_hosting\agent_framework_foundry_hosting\_responses.py packages\foundry_hosting\tests\test_responses.py
git diff --check

Result: Foundry Hosting response tests passed; ruff, format, py_compile, and diff-check passed.