chore: chatroom forbide to read.

Realtime/Realtime/Conversation/LCIMChatRoom.cs

@@ -28,5 +28,9 @@ namespace LeanCloud.Realtime {
         public override Task<LCIMPartiallySuccessResult> AddMembers(IEnumerable<string> clientIds) {
             throw new Exception("Add members is not allowed in chat room.");
         }
+
+        public override Task Read() {
+            return Task.CompletedTask;
+        }
     }
 }

Realtime/Realtime/Conversation/LCIMConversation.cs

@@ -181,7 +181,7 @@ namespace LeanCloud.Realtime {
         /// </summary>
         /// <param name="message"></param>
         /// <returns></returns>
-        public async Task Read() {
+        public virtual async Task Read() {
             if (LastMessage == null) {
                 return;
             }