```

test(chat): 更新消息发送命令处理器测试配置

- 移除 MongoDB 集合模拟，改为使用真实数据库连接
- 添加本地 MongoDB 测试连接字符串常量
- 在静态构造函数中初始化 MongoDB.Entities 静态连接
- 简化构建处理器方法，移除不必要的 Mock 对象
- 更新注释说明测试策略从模拟转向真实数据库
```

tests/BuildFlow.IM.Tests/Chat/SendMessageCommandHandlerTests.cs

@@ -6,8 +6,8 @@ using IM.Infrastructure.Mongo;
 using IM.Infrastructure.Persistence;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
-using MongoDB.Bson;
 using MongoDB.Driver;
+using MongoDB.Entities;
 using Moq;
 using Xunit;
 
@@ -18,6 +18,17 @@ namespace BuildFlow.IM.Tests.Chat;
 /// </summary>
 public class SendMessageCommandHandlerTests
 {
+    /// <summary>本地测试 MongoDB 连接串（与集成测试共用，需本地 docker 起 mongo）。</summary>
+    private const string MongoConnection = "mongodb://mongo:mongo123@localhost:27017/im_unit_test?authSource=admin";
+
+    static SendMessageCommandHandlerTests()
+    {
+        // MongoDB.Entities 用静态 DB.Default，进程内一次性初始化（幂等）。
+        // 消息正文落真实 Mongo，但本测试仅断言未读数语义，不校验 Mongo 内容。
+        var dbName = MongoUrl.Create(MongoConnection).DatabaseName!;
+        DB.InitAsync(dbName, MongoClientSettings.FromConnectionString(MongoConnection)).GetAwaiter().GetResult();
+    }
+
     private static readonly Guid SenderId = Guid.Parse("22222222-2222-2222-2222-222222222222");
     private static readonly Guid RecipientA = Guid.Parse("30000000-0000-0000-0000-000000000001");
     private static readonly Guid RecipientB = Guid.Parse("30000000-0000-0000-0000-000000000002");
@@ -149,32 +160,13 @@ public class SendMessageCommandHandlerTests
     }
 
     /// <summary>
-    /// 构造 Handler，MongoDbContext 用 mock 的 IMongoClient/IMongoDatabase/IMongoCollection 装配，
+    /// 构造 Handler。Mongo 消息正文落真实库（静态 DB.Default 已在静态构造函数初始化），
-    /// 使 InsertOneAsync 成为可调用的空操作（消息正文落库逻辑在真实 Mongo 上验证，此处聚焦未读数语义）。
+    /// 此处聚焦未读数语义。默认所有接收者视为离线（驱动通知链路），个别测试可覆盖。
-    /// </summary>
-    /// <summary>
-    /// 构造 Handler，MongoDbContext 用 mock 的 IMongoClient/IMongoDatabase/IMongoCollection 装配，
-    /// 使 InsertOneAsync 成为可调用的空操作（消息正文落库逻辑在真实 Mongo 上验证，此处聚焦未读数语义）。
-    /// 默认所有接收者视为离线（IOnlinePresenceService.FilterOfflineAsync 返回全部），
-    /// 以便测试通知链路；个别测试可覆盖 onlinePresence 的行为。
     /// </summary>
     private static (SendMessageCommandHandler Handler, Mock<IUnreadCounter> UnreadCounter, Mock<IUserInfoService> UserInfo, Mock<IOnlinePresenceService> OnlinePresence, Mock<IImNotificationPublisher> Notification) BuildHandler(ImDbContext db)
     {
         var unreadCounter = new Mock<IUnreadCounter>();
 
-        var messages = new Mock<IMongoCollection<MessageDocument>>();
-        // 模拟 InsertOneAsync 生成 ObjectId（真实 Mongo 在写入时分配），便于断言 LastMessageId 被设置
-        messages.Setup(m => m.InsertOneAsync(It.IsAny<MessageDocument>(), null, It.IsAny<CancellationToken>()))
-                .Callback<MessageDocument, InsertOneOptions, CancellationToken>((doc, _, _) =>
-                    doc.Id = ObjectId.GenerateNewId().ToString());
-        var mongoDatabase = new Mock<IMongoDatabase>();
-        mongoDatabase.Setup(d => d.GetCollection<MessageDocument>("messages", null))
-                     .Returns(messages.Object);
-        var mongoClient = new Mock<IMongoClient>();
-        mongoClient.Setup(c => c.GetDatabase(It.IsAny<string>(), null))
-                   .Returns(mongoDatabase.Object);
-        var mongoDb = new MongoDbContext(mongoClient.Object, "im-test");
-
         var userInfo = new Mock<IUserInfoService>();
         userInfo.Setup(u => u.GetUsersAsync(It.IsAny<IEnumerable<Guid>>(), It.IsAny<CancellationToken>()))
                 .ReturnsAsync(new Dictionary<string, (string, string?)>());
@@ -187,7 +179,7 @@ public class SendMessageCommandHandlerTests
         var notification = new Mock<IImNotificationPublisher>();
 
         var handler = new SendMessageCommandHandler(
-            db, mongoDb, userInfo.Object, unreadCounter.Object, onlinePresence.Object, notification.Object);
+            db, userInfo.Object, unreadCounter.Object, onlinePresence.Object, notification.Object);
         return (handler, unreadCounter, userInfo, onlinePresence, notification);
     }