Return the chat's live (non-terminal) build plan, or 404.

Authorization reuses load_owned_assistant_chat — the SAME check /message and /refine share — rather than a new ownership check on this route. That collapses the same three non-owner-reachable refusals (absent / cross-tenant / suppressed, wrong chat type, not-yours) into the identical 404 {"detail": "Chat not found"} this surface's siblings answer. When the chat IS the caller's own but carries no live plan, the response is the SAME body — "no plan" and "not your chat" must stay indistinguishable to a non-owner probing for chat existence.