dev_fix: update agent model path to use fully qualified Vertex AI path

rag_agent/agent.py

@@ -38,8 +38,14 @@ vector_search_tool = VectorSearchTool(
 )
 
 # Create agent with vector search tool
+# Configure model with Vertex AI fully qualified path
+model_path = (
+    f'projects/{settings.project_id}/locations/{settings.location}/'
+    f'publishers/google/models/{settings.agent_language_model}'
+)
+
 root_agent = Agent(
-    model=settings.agent_language_model,
+    model=model_path,
     name=settings.agent_name,
     description='A helpful assistant for user questions.',
     instruction=settings.agent_instructions,