fix: ignore empty stream output items

[he-yufeng]

Fixes #3125.

Summary

• Guard the Responses stream accumulator when a malformed response.output_item.added event has no item.
• Leave the current response snapshot unchanged instead of crashing before user code can handle the stream.
• Add a regression test for the malformed event shape.

To verify

• $env:PYTHONPATH=(Resolve-Path .\src).Path; python -m pytest tests\lib\responses\test_responses.py -q
• $env:PYTHONPATH=(Resolve-Path .\src).Path; python -m ruff check src\openai\lib\streaming\responses\_responses.py tests\lib\responses\test_responses.py
• $env:PYTHONPATH=(Resolve-Path .\src).Path; python -m py_compile src\openai\lib\streaming\responses\_responses.py tests\lib\responses\test_responses.py
• git diff --check