add(conversation):完善私聊好友会话服务

backend/IM_API/Dtos/ConversationDto.cs

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+using IM_API.Models;
+
+namespace IM_API.Dtos
+{
+    public class ConversationDto
+    {
+        public int Id { get; set; }
+
+        /// <summary>
+        /// 用户
+        /// </summary>
+        public int UserId { get; set; }
+
+        /// <summary>
+        /// 对方ID（群聊为群聊ID，单聊为单聊ID）	
+        /// </summary>
+        public int TargetId { get; set; }
+
+        /// <summary>
+        /// 最后一条未读消息ID	
+        /// </summary>
+        public int? LastReadMessageId { get; set; }
+
+        /// <summary>
+        /// 未读消息数	
+        /// </summary>
+        public int UnreadCount { get; set; }
+
+        public int ChatType { get; set; }
+
+
+        /// <summary>
+        /// 最后一条最新消息
+        /// </summary>
+        public string LastMessage { get; set; } = null!;
+        /// <summary>
+        /// 最后一条消息时间
+        /// </summary>
+        public DateTime? DateTime { get; set; } = null;
+
+        /// <summary>
+        /// 对方昵称
+        /// </summary>
+        public string TargetName { get; set; }
+        /// <summary>
+        /// 对方头像
+        /// </summary>
+        public string? TargetAvatar { get; set; }
+        public virtual Message? LastReadMessage { get; set; }
+    }
+}