phase3(foundation): http_util dio5 migration + null-safe models

- http_util.dart: dio4->5 (DioException, nullable params, drop unused statusCode,
  null-safe e.response!.data), countryCode ?? ''
- models/* (39 files): make all instance fields nullable (fromJson initializer-list
  pattern forces nullable); fix internal method usages (cart_info/cart_line_item/
  order/shipping_rate) with !/??
- models/ now passes dart analyze with 0 errors

Widget-level null-safety + remaining utils/store continue in next batch.

lib/models/cart_info.dart

@@ -7,18 +7,18 @@ import 'cart_line_item.dart';
 import 'extra_fee.dart';
 
 class CartInfo {
-  int id;
-  double originPrice;
-  double discountPrice;
-  double totalPrice;
-  Business businessInfo;
-  List<CartLineItem> productList;
-  List<ExtraFee> extraFeeList;
-  List<ExtraFee> discountList;
-  int deliveryMethod;
-  String shippingMethod;
-  double amountPaid;
-  String extraData;
+  int? id;
+  double? originPrice;
+  double? discountPrice;
+  double? totalPrice;
+  Business? businessInfo;
+  List<CartLineItem>? productList;
+  List<ExtraFee>? extraFeeList;
+  List<ExtraFee>? discountList;
+  int? deliveryMethod;
+  String? shippingMethod;
+  double? amountPaid;
+  String? extraData;
 
   CartInfo.fromJson(Map<String, dynamic> json)
     : id = json['id'],
@@ -53,7 +53,7 @@ class CartInfo {
     if (productList == null) {
       productList = [item];
     } else {
-      productList.add(item);
+      productList!.add(item);
     }
 
   }
@@ -74,19 +74,19 @@ class CartInfo {
     originPrice = 0.0;
     discountPrice = 0.0;
     totalPrice = 0.0;
-    for (var i = 0; i < productList.length; i++) {
-      newTotal += productList[i].getTotalPrice();
+    for (var i = 0; i < (productList?.length ?? 0); i++) {
+      newTotal += productList![i].getTotalPrice();
     }
     originPrice = newTotal;
     totalPrice = newTotal;
-    return totalPrice;
+    return totalPrice ?? 0.0;
   }
 
   int getProductListTotalQuantity() {
     int qty = 0;
-    if (productList != null && productList.length > 0) {
-      for (var i = 0; i < productList.length; i++) {
-        qty += productList[i].quantity.round();
+    if (productList != null && productList!.length > 0) {
+      for (var i = 0; i < productList!.length; i++) {
+        qty += (productList![i].quantity ?? 0).round();
       }
     }
     return qty;