fix: Mark discount on cart root node as used too

[sveneberth]

Overview

This pull request resolves an issue where discounts applied to the root node of a cart were not considered in the discount handling process. It ensures that all discounts, including those on the cart's root node, are correctly tracked and marked as used.

Key Changes

1. Add Root Node to Discount Collection:

   • The get_discounts_from_cart method now explicitly adds the cart's root node to the list of nodes, ensuring discounts on the root node are correctly included:

     nodes.append(self.shop.cart.skel(skelType="node").read(cart_key))  # the root node itself

2. Ensure Discount Usage is Updated:

   • The mark_discount_used method is updated to handle order_skel instances properly. It tracks and increases quantity_used for discounts associated with an ordered cart.

Impact

These changes ensure that all discounts, including those applied at the root level, are tracked and managed accurately. This fixes inconsistencies in discount processing and improves overall reliability of the cart system.