submodule对gitea不管用，所以直接拉了一份拉格兰

Lagrange.Core/Lagrange.OneBot/Message/Entity/AtSegment.cs

@@ -0,0 +1,33 @@
+using System.Text.Json.Serialization;
+using Lagrange.Core.Message;
+using Lagrange.Core.Message.Entity;
+
+namespace Lagrange.OneBot.Message.Entity;
+
+[Serializable]
+public partial class AtSegment(uint at, string? name)
+{
+    public AtSegment() : this(0, null) { }
+    
+    public AtSegment(uint at) : this(at, null) { }
+    
+    [JsonPropertyName("qq")] [CQProperty] public string At { get; set; } = at.ToString();
+    
+    [JsonPropertyName("name")] [CQProperty] public string? Name { get; set; } = name;
+}
+
+[SegmentSubscriber(typeof(MentionEntity), "at")]
+public partial class AtSegment : SegmentBase
+{
+    public override void Build(MessageBuilder builder, SegmentBase segment)
+    {
+        if (segment is AtSegment atSegment) builder.Mention(atSegment.At == "all" ? 0 : uint.Parse(atSegment.At), atSegment.At == "all" ? "@\u5168\u4f53\u6210\u5458" : null);
+    }
+
+    public override SegmentBase FromEntity(MessageChain chain, IMessageEntity entity)
+    {
+        if (entity is not MentionEntity mentionEntity) throw new ArgumentException("Invalid entity type.");
+
+        return new AtSegment(mentionEntity.Uin, mentionEntity.Name);
+    }
+}