fix(streaming): handle top-level SSE error messages

[raashish1601]

Summary

• raise streaming API errors for explicit event: error SSE payloads even when the message is top-level in the JSON object
• keep the existing nested error.message fallback for backward-compatible streaming payloads
• add focused sync/async regressions in tests/test_streaming.py

Fixes #2487.

Testing

• PYTHONPATH=src python -m pytest -o addopts='' tests/test_streaming.py -q
• PYTHONPATH=src python -m pytest -o addopts='' tests/test_streaming.py -k "error_event_uses_top_level_message or error_event_keeps_nested_error_message_fallback" -q
• python -m ruff check src/openai/_streaming.py tests/test_streaming.py
• python -m py_compile src/openai/_streaming.py tests/test_streaming.py
• git diff --check

[marcuswood-oai]

Thanks for the fix and tests! We’re consolidating this in #3934, which covers the same streaming-error handling on the current transport code. We’ll keep your error-body checks in scope there. Closing this one to keep review in one place.