Minor rounding change for custom line item discounts in POS 11.5

Shopify POS 11.5 changes how fixed-amount custom line item discounts are calculated internally. Instead of applying the discount across the entire line, it will now be applied on a per-unit basis. This may cause rounding differences of up to one cent for amounts that don't divide evenly by quantity (e.g., a $5.00 discount on 3 items becomes $5.01). No action is required for extensions using API version 2026-04 or earlier, but a future API version will require passing per-unit discount amounts directly.