fix: use sync get_state() for SqliteSaver compatibility (#519)

SqliteSaver does not support async methods like aget_state().
Use asyncio.to_thread() to run the sync get_state() call from
async context, maintaining compatibility with the existing
sync graph invocations.

Closes #509

open_notebook/utils/graph_utils.py

@@ -1,11 +1,16 @@
+import asyncio
+
 from langchain_core.runnables import RunnableConfig
 from loguru import logger
 
+
 async def get_session_message_count(graph, session_id: str) -> int:
     """Get message count from LangGraph state, returns 0 on error."""
     try:
-        thread_state = await graph.aget_state( # async version
+        # Use sync get_state() in a thread (SqliteSaver doesn't support async)
-            config=RunnableConfig(configurable={"thread_id": session_id})
+        thread_state = await asyncio.to_thread(
+            graph.get_state,
+            config=RunnableConfig(configurable={"thread_id": session_id}),
         )
         if (
             thread_state