test(openai): add vcr regression coverage for stream helper

[Abhijeet Prasad (AbhiPrasad)]

AI Summary

Add VCR regression coverage for OpenAI stream helper APIs.

This PR extends the existing chat.completions.stream() coverage to also test responses.stream() in both sync and async flows. The new tests verify that:

• unwrapped clients preserve normal SDK behavior and emit no Braintrust spans
• wrapped clients preserve the helper interface and do emit Braintrust spans
• streamed output is still consumable through the helper APIs
• final accumulated responses remain accessible after streaming completes

This covers regression cases related to:

• OpenAI: chat.completions.stream() helper method not instrumented #114 — chat.completions.stream()
• OpenAI: responses.stream() helper method not instrumented #170 — responses.stream()

What changed

Added tests

• sync + async VCR tests for responses.stream()
• assertions that helper APIs still work end-to-end:
  • streamed events can be consumed
  • get_final_response() still works
  • tracing spans are emitted only for wrapped clients

Added cassettes

• test_openai_responses_stream_helper.yaml
• test_openai_responses_stream_helper_async.yaml

Why

responses.stream() is a first-class OpenAI SDK helper and should stay covered by regression tests the same way chat.completions.stream() is. Current instrumentation works through the existing wrapped streaming path, but there was no explicit VCR coverage protecting that behavior.

Test strategy

Used a red/green approach:

• Red: temporarily removed the relevant wrapping path and confirmed the new stream-helper tests failed with zero spans emitted
• Green: restored the current implementation and confirmed the tests passed

Validation

Targeted test run:

cd py && .nox/test_openai-latest/bin/pytest -p no:braintrust \
  src/braintrust/integrations/openai/test_openai.py \
  -k "responses_stream_helper or openai_chat_streaming_sync or openai_chat_streaming_async"

Passed:

• test_openai_responses_stream_helper
• test_openai_responses_stream_helper_async
• existing chat stream helper coverage

[starfolkai]

Drop into this review session: sfk devbox connect pr-214-braintrust-sdk-python-a8ca --attach

No serious bugs or issues found 🎉