Update import (langfuse.callback -> langfuse.langchain) to reflect new API

unit2/langgraph/mail_sorting.ipynb

@@ -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()"