Update LangGraphAgent to use gemma-3-27b-it model instead of gemini-2.5-flash

agent.py

@@ -75,7 +75,7 @@ class LangGraphAgent:
         # Choose an LLM if not provided
         if model is None:
             model = ChatGoogleGenerativeAI(
-                model="gemini-2.5-flash",
+                model="gemma-3-27b-it",
             )
             if model is None and ChatOpenAI is not None:
                 model = ChatOpenAI(