[FIX] delivery_auto_refresh: avoid singleton

If there were more than one delivery line, we'd get a singleton error
when trying to refresh due to the need of taking the discount of the
former lines.

TT35025

delivery_auto_refresh/models/sale_order.py

@@ -61,7 +61,7 @@ def write(self, vals):
             for order in self:
                 delivery_line = order.order_line.filtered("is_delivery")
                 order.with_context(
-                    delivery_discount=delivery_line.discount,
+                    delivery_discount=delivery_line[-1:].discount,
                 )._auto_refresh_delivery()
         return res