feat(model): 添加聊天音频产品模型

- 创建 ChatAudioProductModel 类定义产品数据结构
- 实现 fromJson 工厂构造函数支持 JSON 反序列化
- 实现 toJson 方法支持对象序列化为 JSON
- 添加 isFreeProduct 计算属性判断是否为免费产品
- 定义产品基本信息字段包括 ID、标题、价格等
- 添加字符串表示方法便于调试和日志输出

lib/model/rtc/chat_audio_product_model.dart

@@ -0,0 +1,53 @@
+/// 聊天音频产品模型
+class ChatAudioProductModel {
+  final String isFree; // "true" 或 "false"
+  final String productId;
+  final String productSpecId;
+  final int mainCategory;
+  final int subCategory;
+  final String productTitle;
+  final double unitSellingPrice;
+
+  ChatAudioProductModel({
+    required this.isFree,
+    required this.productId,
+    required this.productSpecId,
+    required this.mainCategory,
+    required this.subCategory,
+    required this.productTitle,
+    required this.unitSellingPrice,
+  });
+
+  factory ChatAudioProductModel.fromJson(Map<String, dynamic> json) {
+    return ChatAudioProductModel(
+      isFree: json['isFree']?.toString() ?? 'false',
+      productId: json['productId']?.toString() ?? '',
+      productSpecId: json['productSpecId']?.toString() ?? '',
+      mainCategory: json['mainCategory'] as int? ?? 0,
+      subCategory: json['subCategory'] as int? ?? 0,
+      productTitle: json['productTitle']?.toString() ?? '',
+      unitSellingPrice: (json['unitSellingPrice'] as num?)?.toDouble() ?? 0.0,
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'isFree': isFree,
+      'productId': productId,
+      'productSpecId': productSpecId,
+      'mainCategory': mainCategory,
+      'subCategory': subCategory,
+      'productTitle': productTitle,
+      'unitSellingPrice': unitSellingPrice,
+    };
+  }
+
+  /// 判断是否为免费产品
+  bool get isFreeProduct => isFree.toLowerCase() == 'true';
+
+  @override
+  String toString() {
+    return 'ChatAudioProductModel(productId: $productId, productTitle: $productTitle, unitSellingPrice: $unitSellingPrice, isFree: $isFree)';
+  }
+}
+