fix(langgraph): Add missing SystemMessage and HumanMessage import

Co-Authored-By: Claude <noreply@anthropic.com>

Author: priscilawebdev

docs/platforms/javascript/common/configuration/integrations/langgraph.mdx

@@ -103,6 +103,7 @@ The `instrumentLangGraph` helper adds instrumentation for [`@langchain/langgraph
 ```javascript
 import { ChatOpenAI } from "@langchain/openai";
 import { StateGraph, MessagesAnnotation, START, END } from '@langchain/langgraph';
+import { SystemMessage, HumanMessage } from '@langchain/core/messages';
 
 // Create LLM call
 const llm = new ChatOpenAI({