fix: ensure tool parameters are properly handled by reloading JSON

--bug=1053980 --user=刘瑞斌 【应用】MCP节点的工具参数引用参数时，对话过程中参数值发生变化，MCP节点没有获取最新的参数值 https://www.tapd.cn/57709429/s/1676830

Author: liuruibin

apps/application/flow/step_node/mcp_node/impl/base_mcp_node.py

@@ -18,7 +18,8 @@ def save_context(self, details, workflow_manage):
 
     def execute(self, mcp_servers, mcp_server, mcp_tool, tool_params, **kwargs) -> NodeResult:
         servers = json.loads(mcp_servers)
-        params = self.handle_variables(tool_params)
+        params = json.loads(json.dumps(tool_params))
+        params = self.handle_variables(params)
 
         async def call_tool(s, session, t, a):
             async with MultiServerMCPClient(s) as client: