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/ticket.dart

@@ -4,10 +4,10 @@ import 'dart:convert';
 import 'simple_business.dart';
 
 class TicketFile {
-  int id;
-  String url;
-  String fileName;
-  String fileType;
+  int? id;
+  String? url;
+  String? fileName;
+  String? fileType;
 
   TicketFile.fromJson(Map<String, dynamic> json)
     : id = json['id'],
@@ -29,9 +29,9 @@ class TicketFile {
 }
 
 class Issue {
-  int id;
-  String msg;
-  List<TicketFile> files;
+  int? id;
+  String? msg;
+  List<TicketFile>? files;
 
   Issue.fromJson(Map<String, dynamic> json)
     : id = json['id'],
@@ -51,11 +51,11 @@ class Issue {
 }
 
 class FollowUp {
-  int id;
-  String msg;
-  String poster;
-  String createdAt;
-  List<TicketFile> files;
+  int? id;
+  String? msg;
+  String? poster;
+  String? createdAt;
+  List<TicketFile>? files;
 
   FollowUp.fromJson(Map<String, dynamic> json)
     : id = json['id'],
@@ -79,12 +79,12 @@ class FollowUp {
 }
 
 class Ticket {
-  int id;
-  Issue issue;
-  Store store;
-  List<FollowUp> followUps;
-  bool isClosed;
-  String createdAt;
+  int? id;
+  Issue? issue;
+  Store? store;
+  List<FollowUp>? followUps;
+  bool? isClosed;
+  String? createdAt;
 
   Ticket.fromJson(Map<String, dynamic> json)
     : id = json['id'],