code: def test_system_message(self):
      # Test default system message
      self.assertEqual(
          self.agent.system_message,
          [
              {
                  "type": "text",
                  "text": "You are a helpful AI assistant.",
              }
          ],
      )
  
      # Test updating system message
      new_message = f"We will discuss <img {base64_encoded_image}> in this conversation."
      self.agent.update_system_message(new_message)
      self.assertEqual(
          self.agent.system_message,
          [
              {"type": "text", "text": "We will discuss "},
              {
                  "type": "image_url",
                  "image_url": {"url": base64_encoded_image},
              },
              {"type": "text", "text": " in this conversation."},
          ],
      )
  
file_path: autogen/test/agentchat/contrib/test_lmm.py
imports: import unittest
  from unittest.mock import MagicMock
  import pytest
  import autogen
  from autogen.agentchat.agent import Agent
label_desc: Use image node_id 2 for calling the TestMultimodalConversableAgent obj's underlying member method code with example usage: obj.test_system_message() without return types