chore: add returned type in assistant(state: AgentState)

agents/assistant_agent.py

@@ -148,7 +148,7 @@ def build_langgraph(
     # [4] Assistant Node
     # ---
 
-    def assistant(state: AgentState):
+    def assistant(state: AgentState) -> AgentState:
         file = state["input_file"]
         sys_msg = SystemMessage(
             content=f"""