Fix responses stream null output item handling

[fengjikui]

Fixes #3125.

Summary

ResponseStreamState.accumulate_event() assumes every response.output_item.added event has an item. Some OpenAI-compatible Responses streams can send item: null; when that happens the SDK currently raises before user code can recover:

AttributeError: 'NoneType' object has no attribute 'type'

This treats a null added item as a no-op for the accumulated snapshot, matching the fact that there is no concrete output item to append.

Validation

I reproduced the failure on current origin/main by feeding ResponseStreamState.handle_event() a normal response.created event followed by response.output_item.added with item=None; current main raises the AttributeError above.

On this branch:

• uv run --with pytest --with pytest-asyncio --with respx --with inline-snapshot pytest -o addopts='' tests/lib/responses/test_responses.py::test_response_stream_ignores_null_output_item_added -q -> 1 passed in 0.12s
• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run --with pytest --with pytest-asyncio --with respx --with inline-snapshot pytest -o addopts='' tests/lib/responses/test_responses.py -q -> 6 passed in 0.87s
• uv run --with ruff ruff check src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_responses.py -> All checks passed!
• uv run python -m py_compile src/openai/lib/streaming/responses/_responses.py tests/lib/responses/test_responses.py -> passed
• git diff --check -> clean