Docs: note chat endpoints are REST-only in OpenAPI README

[yaront1111]

Context

The OpenAPI output is generated from protobufs and does not currently include the REST-only run chat endpoints.

Task

Document this limitation in docs/api/openapi/README.md so users know where to find chat endpoint docs.

Acceptance criteria

• Add a short note that run chat endpoints are REST-only and documented in docs/api.md.
• Keep the note concise (2–3 lines).