fix(openai): trace chat streaming audio attachments

Preserve chat completion delta.audio fields in streamed OpenAI spans and
materialize the audio payload as a Braintrust attachment instead of
logging raw base64 data.

Also apply the same attachment materialization to non-streaming chat
completion audio output and add VCR-backed regression coverage for the
streaming path plus focused unit tests for the audio aggregation logic.

Author: AbhiPrasad