[FIX] point_of_sale: correct compute fixed tax negative quantity

Have a product with a fixed amount tax.
Make a "return" of that product in the pos (i.e. negative quantity)

Before this commit:
- The amount of the return was `tax_amount + product_amount` instead of `-1 * positive_total_amount`
- The amount differed from what can be observed in sales

This was because the sign of the quantity was applied twice

After this commit,:
- the amounts of positive and negative amounts are symmetrical
- they match the behavior in sale

The logic is very similar to bb72dea98d

OPW 2026278

closes odoo/odoo#34548

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>

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

@@ -1706,8 +1706,11 @@ exports.Orderline = Backbone.Model.extend({
     },
     _compute_all: function(tax, base_amount, quantity) {
         if (tax.amount_type === 'fixed') {
-            var sign_base_amount = base_amount >= 0 ? 1 : -1;
-            return (Math.abs(tax.amount) * sign_base_amount) * quantity;
+            var sign_base_amount = Math.sign(base_amount) || 1;
+            // Since base amount has been computed with quantity
+            // we take the abs of quantity
+            // Same logic as bb72dea98de4dae8f59e397f232a0636411d37ce
+            return tax.amount * sign_base_amount * Math.abs(quantity);
         }
         if ((tax.amount_type === 'percent' && !tax.price_include) || (tax.amount_type === 'division' && tax.price_include)){
             return base_amount * tax.amount / 100;