OpenAI endpoint stage 10.2 — n-choice and logprobs compatibility

Minor patch release on top of stage 10.1.\n\nHighlights:\n- Adds Chat Completions support for n choices up to 16.\n- Adds Chat Completions logprobs/top_logprobs response shapes, including streaming chunk shape when requested.\n- Adds legacy Completions n choices and logprobs response shapes.\n- Offsets seed per choice for reproducible multi-choice sampled requests.\n- Expands OpenAI Python SDK smoke coverage for n/logprobs parsing.\n- Documents that HTTP logprob values are compatibility placeholders until the adapter exposes exact per-token logits.\n\nVerification:\n- python3 -m pytest tests/test_openai_api.py tests/test_openai_compat_payloads.py tests/test_openai_sdk_smoke.py tests/test_sampling_controls.py -q -> 39 passed, 2 skipped, 1 warning.\n- PYTHONPATH=/Users/taewoopark/personal/DRIFT /tmp/drift-openai-sdk-venv/bin/python -m pytest tests/test_openai_api.py tests/test_openai_compat_payloads.py tests/test_openai_sdk_smoke.py tests/test_sampling_controls.py -q -> 40 passed, 1 skipped, 1 warning.\n- python3 -m compileall drift tests scripts/openai_compat_smoke.py -> passed.\n- git diff --check -> passed.\n- CLI help spot checks: run/node/serve/bench/parity passed.\n\nKnown scope notes:\n- DRIFT now emits OpenAI-compatible logprobs objects for client compatibility, but values are placeholders rather than exact model log probabilities.\n- Tool execution, full constrained decoding, audio, and multimodal content remain outside this text-generation PR scope.