fix: move database service call inside session context in workflow draft variable API (#23996)

api/controllers/console/app/workflow_draft_variable.py

             draft_var_srv = WorkflowDraftVariableService(
                 session=session,
             )
-        workflow_vars = draft_var_srv.list_variables_without_values(
-            app_id=app_model.id,
-            page=args.page,
-            limit=args.limit,
-        )
+            workflow_vars = draft_var_srv.list_variables_without_values(
+                app_id=app_model.id,
+                page=args.page,
+                limit=args.limit,
+            )
 
         return workflow_vars