Change default provider in build_graph function from 'groq' to 'google'

agent.py

@@ -51,7 +51,7 @@ tools = [
 ]
 
 # Build graph function
-def build_graph(provider: str = "groq"):
+def build_graph(provider: str = "google"):
     """Build the graph"""
     # Load environment variables from .env file
     if provider == "google":