Update unit2/langgraph/mail_sorting.ipynb

1) adding import for START which was missing
2) correcting callbackhandler import to v3 `from langfuse.langchain import CallbackHandler`

unit2/langgraph/mail_sorting.ipynb

@@ -50,7 +50,7 @@
    "source": [
     "import os\n",
     "from typing import TypedDict, List, Dict, Any, Optional\n",
-    "from langgraph.graph import StateGraph, END\n",
+    "from langgraph.graph import StateGraph, START, END\n",
     "from langchain_openai import ChatOpenAI\n",
     "from langchain_core.messages import HumanMessage\n",
     "\n",
@@ -361,7 +361,7 @@
    "outputs": [],
    "execution_count": null,
    "source": [
-    "from langfuse.callback import CallbackHandler\n",
+    "from langfuse.langchain import CallbackHandler\n",
     "\n",
     "# Initialize Langfuse CallbackHandler for LangGraph/Langchain (tracing)\n",
     "langfuse_handler = CallbackHandler()"