/v1/ocr names its engine in a multipart field called `engine`, not `model`.
The gateway only ever looked for `model`, so an OCR request carried no model as
far as it was concerned: per-model placement resolved correctly, the test
reported 'target: its own pod', and then the gateway refused with 'nothing
configures ocr to run remotely'. Two layers disagreeing about the same request.

_MODEL_FIELD maps the endpoints whose field is named something else, and the
extractor takes the field name for JSON, multipart and urlencoded alike.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY