fix(openai): keep streamed choices separate - #1853

What does this PR do?

OpenAI streams with n > 1 return separate choices to the application, but Langfuse merges their recorded content and tool arguments. This keeps each choice separate in index order and preserves the single-choice output shape.

The parser accumulates streamed deltas by choice index, which keeps tool-call indexes local to each choice. The sync and async regressions decode the actual exported OTLP payload after consuming an interleaved HTTP stream, including its usage-only final chunk.

Fixes langfuse/langfuse#16998

Type of change

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

Verification

The run used Python 3.12.13, Langfuse 4.15.1, OpenAI 2.29.0 and HTTPX 0.28.1. Both endpoints run locally; this is not a live-provider or deployed-server result.

uv run --frozen pytest -q -s tests/unit/test_openai.py::test_streaming_chat_completion_keeps_multiple_choices_separate

E AssertionError: {'content': 'BA10', 'role': 'assistant', 'tool_calls': [{'function': {'arguments': '{"value":"B{"value":"A0"}1"}', 'name': 'lookup'}, 'id': 'call-0', 'type': 'function'}]}
exit=1

{"output":[{"role":"assistant","tool_calls":[{"function":{"name":"lookup","arguments":"{\"value\":\"A0\"}"},"id":"call-0","type":"function"}],"content":"A0"},{"role":"assistant","tool_calls":[{"function":{"name":"lookup","arguments":"{\"value\":\"B1\"}"},"id":"call-1","type":"function"}],"content":"B1"}],"usage":{"completion_tokens":5,"prompt_tokens":7,"total_tokens":12,"completion_tokens_details":null,"prompt_tokens_details":null}}
============================== 2 passed in 3.74s ===============================
exit=0

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.