feat(openai): capture audio transcript deltas in streaming chat completions - #1831

• Update _extract_streamed_openai_response to inspect delta.audio for streaming chunks
• Extract and concatenate transcript tokens into completion["content"]
• Ensure text output is captured in Langfuse dashboard traces when using audio modalities

What does this PR do?

When streaming chat completions with audio modality enabled (modalities=["text", "audio"]), OpenAI returns spoken transcript chunks inside chunk.choices[0].delta.audio["transcript"] instead of chunk.choices[0].delta.content. Currently, this leaves the extracted completion content empty in Langfuse traces. This PR updates _extract_streamed_openai_response() to capture and concatenate transcript deltas into completion["content"].

Fixes #

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation update
• Tooling, CI, or repo maintenance

Verification

List the main commands you ran: pytest tests/unit/test_openai.py

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed.
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

### Issue 1
langfuse/openai.py:804-805
**Streaming transcript path untested**
The new object/dictionary normalization and multi-chunk transcript concatenation have no automated coverage, so regressions can silently restore empty or malformed Langfuse trace output.
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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

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Context used:

• Knowledge Base — OpenAI instrumentation