feat: validate discount date in payment schedule (backport #44646) (#44726)

Co-authored-by: barredterra <14891507+barredterra@users.noreply.github.com>

erpnext/controllers/accounts_controller.py

@@ -2188,6 +2188,9 @@ class AccountsController(TransactionBase):
 			return
 
 		for d in self.get("payment_schedule"):
+			if d.due_date and d.discount_date:
+				d.validate_from_to_dates("discount_date", "due_date")
+
 			if self.doctype == "Sales Order" and getdate(d.due_date) < getdate(self.transaction_date):
 				frappe.throw(
 					_("Row {0}: Due Date in the Payment Terms table cannot be before Posting Date").format(