feat: update split bill

lib/data/models/request/payment_request.dart

@@ -121,17 +121,17 @@ class PaymentSplitBillRequest {
 
 class SplitItem {
   final String orderItemId;
-  final int amount;
+  final int quantity;
 
   SplitItem({
     required this.orderItemId,
-    required this.amount,
+    required this.quantity,
   });
 
   Map<String, dynamic> toJson() {
     return {
       'order_item_id': orderItemId,
-      'amount': amount,
+      'quantity': quantity,
     };
   }
 }