Translate Responses API requests (input→messages, instructions→system,
max_output_tokens→max_tokens, text.format→response_format, flat tools→nested
{function:{…}}) to upstream chat/completions, then convert responses back to
Responses format (streaming SSE event lifecycle + non-streaming JSON).
Verified against OpenAI migration guide and Python SDK Response model:
- Echo back required fields parallel_tool_calls/tool_choice/tools
- Include content:[] in reasoning items, logprobs:[] in output_text parts
- Support function_call/function_call_output multi-turn input items
- Map usage fields prompt_tokens→input_tokens, completion_tokens→output_tokens
