Fixed tax subrate calculation for Line Items and Custom Line Items

We fixed the calculation and rounding of tax subrates on (Custom) Line Items to provide a more consistent experience.

Previously, individual (Custom) Line Item taxes were calculated once based on the sum of the combined subrates. If the calculation of a subrate resulted in a fractional tax amount, it could cause discrepancies between the total price of the (Custom) Line Item(s) and the total price of the Cart.

Now, each tax subrate is calculated and rounded separately. This change ensures that the sum of the tax portions is identical to the (Custom) Line Item's total tax amount. As a result, the total price of all (Custom) Line Items will now match the total price of the Cart.