Fix erase verb not removing chat messages in some cases (#21355)

* Fix erase verb not removing chat messages in some cases

* Admin changelog

* Fix deleting messages with entity id 0

Content.Server/Chat/ChatUser.cs

@@ -0,0 +1,34 @@
+using Content.Shared.Chat;
+
+namespace Content.Server.Chat;
+
+public sealed class ChatUser
+{
+    /// <summary>
+    ///     The unique key associated with this chat user, starting from 1 and incremented.
+    ///     Used when the server sends <see cref="MsgChatMessage"/>.
+    ///     Used on the client to delete messages sent by this user when receiving
+    ///     <see cref="MsgDeleteChatMessagesBy"/>.
+    /// </summary>
+    public readonly int Key;
+
+    /// <summary>
+    ///     All entities that this chat user was attached to while sending chat messages.
+    ///     Sent to the client to delete messages sent by those entities when receiving
+    ///     <see cref="MsgDeleteChatMessagesBy"/>.
+    /// </summary>
+    public readonly HashSet<NetEntity> Entities = new();
+
+    public ChatUser(int key)
+    {
+        Key = key;
+    }
+
+    public void AddEntity(NetEntity entity)
+    {
+        if (!entity.Valid)
+            return;
+
+        Entities.Add(entity);
+    }
+}