The existing /v1/responses shim returned Chat Completions format, which
broke OpenAI SDK clients using openai.responses.create(). This commit
replaces the endpoint with a proper implementation that:
- Returns `output` array with `output_text` content parts instead of
`choices` with `message`
- Uses `input_tokens`/`output_tokens` instead of `prompt_tokens`/
`completion_tokens` in usage
- Sets `object: "response"` and `id: "resp_..."`
- Emits named SSE events for streaming (response.created,
response.output_text.delta, response.completed, etc.)
- Accepts all OpenAI Responses API fields (tools, store, metadata,
previous_response_id) without erroring -- silently ignored
- Maps `developer` role to `system` and `input_text`/`input_image`
content parts to the internal Chat format
Adds Pydantic schemas for request/response models and 23 unit tests
covering schema validation, input normalisation, and response format.
