refactor(chess): 重构象棋程序基础结构,使用Nullable重构核心代码

- 更新了多个文件的代码结构和类型定义，提高了代码的健壮性和可维护性
- 优化了事件处理、棋子管理和移动逻辑，为后续功能扩展打下坚实基础
- 修复了一些潜在的空指针异常问题，提高了程序的稳定性
- 修复部分bug

Scripts/Lib/RPHelper.cs

@@ -9,12 +9,14 @@ public static class RPHelper
         return message.ToDictionary();
     }
 
-    public static RPMessage DeserializeRPMessage(Dictionary data)
+    public static RPMessage? DeserializeRPMessage(Dictionary data)
     {
-        return new RPMessage
-        {
-            Type    = data.ContainsKey("type") ? (string)data["type"] : null,
-            Cmd     = data.ContainsKey("cmd") ? (string)data["cmd"] : null,
+        if (!data.ContainsKey("type") || !data.ContainsKey("cmd")) {
+            return null;
+        }
+        return new RPMessage {
+            Type    = (string)data["type"],
+            Cmd     = (string)data["cmd"],
             Code    = data.ContainsKey("code") ? (string)data["code"] : null,
             Uid     = data.ContainsKey("uid") ? (string)data["uid"] : null,
             Token   = data.ContainsKey("token") ? (string)data["token"] : null,
@@ -28,7 +30,7 @@ public static class RPHelper
         ws.SendJsonEx(RPHelper.SerializeRPMessage(message));
     }
 
-    public static RPMessage HandleIncomingMessage(string jsonMessage)
+    public static RPMessage? HandleIncomingMessage(string jsonMessage)
     {
         var dataDict = Json.ParseString(jsonMessage).AsGodotDictionary();
         if (dataDict != null)