code: def test_mixed_content(self):
      content = [
          {"type": "text", "text": "hello"},
          {"type": "image_url", "url": "http://example.com/image.png"},
      ]
      self.assertEqual(content_str(content), "hello<image>")
  
imports: import importlib.metadata
  import os
  import sys
  import unittest
  import pytest
  import autogen
  from autogen.code_utils import PATH_SEPARATOR, UNKNOWN, WIN32, content_str, execute_code, extract_code, improve_code, improve_function, infer_lang
label_desc: Use image node_id 3 for calling the TestContentStr obj's underlying member method code with example usage: obj.test_mixed_content() without return types