[Core] fix MentionEntity parse (#105)

Author: Szzrain

Lagrange.Core/Message/Entities/MentionEntity.cs

@@ -7,7 +7,7 @@ namespace Lagrange.Core.Message.Entities;
 
 public class MentionEntity(long uin, string? display) : IMessageEntity
 {
-    public long Uin { get; } = uin;
+    public long Uin { get; private set; } = uin;
 
     public string? Display { get; private set; } = display;
 
@@ -40,6 +40,16 @@ async Task IMessageEntity.Preprocess(BotContext context, BotMessage message)
             Uid = contact.Uid;
         }
     }
+
+    Task IMessageEntity.Postprocess(BotContext context, BotMessage message)
+    {
+        if (Uin == 0 && Uid != null)
+        {
+            Uin = context.CacheContext.ResolveUin(Uid);
+        }
+        
+        return Task.CompletedTask;
+    }
 
     string IMessageEntity.ToPreviewString() => Display ?? "";
 
@@ -71,7 +81,12 @@ Elem[] IMessageEntity.Build()
         if (target.Text is { Attr6Buf.Length: > 0, PbReserve: { Length: > 0 } reserve })
         {
             var attr = ProtoHelper.Deserialize<TextResvAttr>(reserve.Span);
-            return new MentionEntity((long)attr.AtMemberUin, target.Text.TextMsg);
+            var entity = new MentionEntity((long)attr.AtMemberUin, target.Text.TextMsg);
+            if (attr.AtType == 2) // AtType: 1 = mention_all, 2 = mention specific user
+            {
+                entity.Uid = attr.AtMemberUid;
+            }
+            return entity;
         }
 
         return null;