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/Subproduct.dart

@@ -4,9 +4,9 @@ import 'dart:convert';
 import 'package:flutter_wisetronic/models/product_attribute.dart';
 
 class Subproduct {
-  int id;
-  SimpleProduct product;
-  double quantity;
+  int? id;
+  SimpleProduct? product;
+  double? quantity;
 
   Subproduct.fromJson(Map<String, dynamic> json) :
       id = json['id'],
@@ -26,15 +26,15 @@ class Subproduct {
 }
 
 class SimpleProduct {
-  int id;
-  String name;
-  String sku;
-  double price;
-  String description;
-  String image;
-  List<ProductAttribute> productAttributes;
-  double customerLastPrice;
-  String extraData;
+  int? id;
+  String? name;
+  String? sku;
+  double? price;
+  String? description;
+  String? image;
+  List<ProductAttribute>? productAttributes;
+  double? customerLastPrice;
+  String? extraData;
 
   SimpleProduct.fromJson(Map<String, dynamic> json) :
       id = json['id'],