Adds opt-in native provider records to OpenAI-compatible streaming.
- Send `"stream_options": {"include_aicodebox_events": true}` with
`stream: true` to receive named `aicodebox.native` SSE events alongside the
standard OpenAI chunks. Existing content chunks and `[DONE]` are unchanged.
- Each native event carries its sequence, retry attempt, adapter backend, event
type, and the original provider object or raw stdout line. This preserves
thinking, tool, and diagnostic records without altering normal streams.
- The option is rejected with `400` unless its value is a boolean and
`stream: true` is set.