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/product_attribute.dart

@@ -4,14 +4,14 @@ import 'dart:convert';
 import 'product_option.dart';
 
 class ProductAttribute {
-  int id;
-  String name;
-  List<ProductOption> productOptions;
-  bool singleSelection;
-  bool byQuantity;
-  bool required;
-  String extra;
-  bool disabled;
+  int? id;
+  String? name;
+  List<ProductOption>? productOptions;
+  bool? singleSelection;
+  bool? byQuantity;
+  bool? required;
+  String? extra;
+  bool? disabled;
 
   ProductAttribute.fromJson(Map<String, dynamic> json)
     : id = json['id'],