chatstate fix

src/chatbot_backend.py

@@ -1,6 +1,6 @@
 # chatbot_backend.py
 from langgraph.graph import StateGraph, START
-from typing import TypedDict, Annotated
+from typing import TypedDict, Annotated, Optional
 from langchain_core.messages import BaseMessage, HumanMessage, SystemMessage
 from langgraph.checkpoint.sqlite import SqliteSaver
 from langgraph.graph.message import add_messages
@@ -24,7 +24,7 @@ tools = [search_tool]
 # -------------------
 class ChatState(TypedDict):
     messages: Annotated[list[BaseMessage], add_messages]
-    doc_content: str | None      # User's uploaded document content
+    doc_content: Optional[str]      # User's uploaded document content
     search_enabled: bool         # Whether to use external search
     model_choice: str            # Model selected ("OpenAI (Paid)" or "LLaMA (Open Source)")
     doc_source: str | None       # Document source ("Upload Your Documents" or "Use Sample Documents")