add initial testing

agent/nodes/tool.ts

@@ -0,0 +1,8 @@
+import type { ToolCall } from "@langchain/core/messages/tool";
+import { task } from "@langchain/langgraph";
+import { arithmeticToolsByName } from "../tools/arithmetic";
+
+export const toolNode = task({ name: "callTool" }, async (toolCall: ToolCall) => {
+  const tool = arithmeticToolsByName[toolCall.name];
+  return tool.invoke(toolCall);
+});