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/distance_info.dart

@@ -4,9 +4,9 @@ import 'dart:convert';
 import 'text_value.dart';
 
 class DistanceInfo {
-  TextValue duration;
-  TextValue distance;
-  String status;
+  TextValue? duration;
+  TextValue? distance;
+  String? status;
 
   DistanceInfo.fromJson(Map<String, dynamic> json)
     : duration = json['duration'] != null ? TextValue.fromJson(json['duration']) : null,