fix(streaming): restore function name on arguments done events

[sylvesterkaczmarek]

Summary

• restore a missing name on response.function_call_arguments.done events from the already accumulated function-call snapshot
• preserve a non-empty function name supplied by the server
• leave raw response schemas and request behaviour unchanged
• add focused regression coverage for both fallback and preservation cases

Fixes #2723

Test plan

• uv run ruff format src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_function_call_done_event.py
• uv run ruff check --fix tests/lib/responses/test_function_call_done_event.py
• uv run ruff check src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_function_call_done_event.py
• uv run pytest -o addopts= -q tests/lib/responses/test_function_call_done_event.py
• git diff --check

All focused verification steps passed before the branch was reduced to its single final commit.