fix(openai): preserve async parsing for streaming responses

[Curtis Galione (cjgalione)]

resolves https://linear.app/braintrustdata/issue/SDK-86/async-streaming-response-parsing-not-preserved

Summary

• await asynchronous raw-response parsing before wrapping OpenAI Responses and Chat Completions streams
• preserve the awaitable parse() contract exposed by with_streaming_response
• extend existing cassette-backed streaming tests to await and consume the traced streams without mocks or new cassette churn

Root cause

openai.AsyncAPIResponse.parse() is asynchronous, but the tracing proxy replaced it with a synchronous method that returned the traced stream directly. The wrappers also attempted to instrument the un-awaited parser coroutine rather than the parsed iterator.

This broke callers such as OpenAI Agents that enter with_streaming_response and then call await api_response.parse(). Chat Completions had the same issue on its async raw streaming path.

The wrappers now detect and await asynchronous parsing before constructing the traced iterator, remember the original parse contract, and expose an awaitable parse() on the raw-response proxy. Synchronous raw-response behavior remains unchanged.

Validation

• reproduced the Chat Completions failure with existing VCR coverage: parse() was not awaitable and the underlying parser coroutine was never awaited
• full test_openai(latest) session with cassette playback: 70 tests passed, plus attachment and OpenRouter gateway suites
• focused cassette playback for OpenAI 1.92.0, 1.77.0, and 1.71.0
• pre-commit hooks: Ruff format/check, cassette consistency, pytest-pin synchronization, codespell, and whitespace checks