Submit an answer and advance the execution
Submits the option the member selected for the current question node and returns the next node to render. If the engine has reached a terminal, `next.kind` is `terminal` and the execution has been transitioned to `completed`. The `nodeId` in the request is checked against the engine’s current node; mismatches return a contract error code and the client should refetch the current node. The `Idempotency-Key` header is REQUIRED. Same key with the same body within the dedup window returns the cached response; same key with a different body returns a conflict error. Empty `branchCode` is permitted on `info` nodes (single implicit branch).