Cancel an order by client order ID

Request to cancel an order by client ID through the public cloid endpoint.

Concrete cancel-order response envelope for the published schema graph.

The cancel handlers return ApiResponse<OrderUpdateMessage> on the wire, but the generic ApiResponse schema loses the inner type (utoipa collapses T to serde_json::Value). This named wrapper keeps data typed as Option<OrderUpdateMessage> so spec-driven clients receive the correct schema.