Cancel a single seller order item on an order. Order items are fully cancelled by default. Order items can be
partially cancelled by using the newQuantity parameter. Note that doing a partial cancellation will cancel the
original seller order item and create a new order item representing the uncancelled quantity. Fulfilled items
cannot be cancelled.
Note: 'Order item' in this context specifically refers to seller order items (found at order.sellerOrders.items).
The top-level order items (found at order.items) are not modified by cancellation endpoints.
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