fix(wxapi): 修复获取图文素材接口模型的字段缺失问题

src/SKIT.FlurlHttpClient.Wechat.Api/Models/CgibinMaterial/CgibinMaterialGetMaterialAsNewsResponse.cs

@@ -19,6 +19,13 @@ namespace SKIT.FlurlHttpClient.Wechat.Api.Models
                 [System.Text.Json.Serialization.JsonPropertyName("thumb_media_id")]
                 public string ThumbnailMediaId { get; set; } = default!;
 
+                /// <summary>
+                /// 获取或设置图文消息缩略图 URL。
+                /// </summary>
+                [Newtonsoft.Json.JsonProperty("thumb_url")]
+                [System.Text.Json.Serialization.JsonPropertyName("thumb_url")]
+                public string ThumbnailUrl { get; set; } = default!;
+
                 /// <summary>
                 /// 获取或设置图文消息标题。
                 /// </summary>

test/SKIT.FlurlHttpClient.Wechat.Api.UnitTests/ModelSamples/CgibinMaterial/CgibinMaterialGetMaterialAsNewsResponse.json

@@ -3,6 +3,7 @@
         {
             "title": "TITLE",
             "thumb_media_id": "THUMB_MEDIA_ID",
+            "thumb_url": "thumb_url",
             "author": "AUTHOR",
             "digest": "DIGEST",
             "show_cover_pic": 1,