OpenAI endpoint stage 10.4 - Responses API streaming

Adds semantic SSE streaming for /v1/responses with response.created, response.output_text.delta, and response.completed events; accepts Responses-style text.format structured output and function tool payloads; expands OpenAI Python SDK smoke coverage.\n\nVerification:\n- python3 -m compileall drift tests scripts/openai_compat_smoke.py\n- python3 -m pytest -q\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\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\n- git diff --check\n- CLI help gates for drift.cli, run, node, serve, bench; parity/itest help gates\n\nHardware/model parity execution was not rerun here because this local Python environment does not have torch installed.