refactor: simplify return statement in get_reference_content method

Author: liuruibin

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

@@ -57,9 +57,9 @@ def handle_variables(self, tool_params):
         return tool_params
 
     def get_reference_content(self, fields: List[str]):
-        return str(self.workflow_manage.get_reference_field(
+        return self.workflow_manage.get_reference_field(
             fields[0],
-            fields[1:]))
+            fields[1:])
 
     def get_details(self, index: int, **kwargs):
         return {