[FIX] point_of_sale: prevent showing downpayment as discount on receipt

Current behavior:
When applying a discount and a downpayment on a product in the PoS
the discount line on the receipt would be the sum of the discount
and the downpayment. But the downpayment is not a discount.

Steps to reproduce:
- Install PoS
- Change discount policy to Show public price & discount to the
  customer, in the public pricelist
- Create a promotion program that applies 10% discount
- Use this promotion program in the PoS
- Create a quotation for any product (e.g with a product that cost
  1000$)
- In the PoS go in the quotation/order and apply a downpayment to
  this quotation.
- Pay the downpayment, and go in the quotation/order again. But this
  time settle the order.
- After the payment on the receipt you will see the discount line
  value is not correct. It's the sum of the discount + the downpayment

opw-2792417

closes odoo/odoo#89213

X-original-commit: fc546e0481ae0159e67791240833bc1e38a4440f
Signed-off-by: Masereel Pierre <pim@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>

addons/point_of_sale/static/src/js/models.js

@@ -2724,6 +2724,9 @@ class Order extends PosModel {
     }
     get_total_discount() {
         return round_pr(this.orderlines.reduce((function(sum, orderLine) {
+            if (orderLine.pos.config.down_payment_product_id[0] === orderLine.product.id) {
+                return 0;
+            }
             sum += (orderLine.get_unit_price() * (orderLine.get_discount()/100) * orderLine.get_quantity());
             if (orderLine.display_discount_policy() === 'without_discount'){
                 sum += ((orderLine.get_lst_price() - orderLine.get_unit_price()) * orderLine.get_quantity());