code: def test_chat_message_list_with_missing_variables(self):
      prompt_builder = DynamicChatPromptBuilder(
          runtime_variables=["documents"]
      )
      prompt_source = [
          ChatMessage.from_user(content="Hello, {{ who }}!")
      ]
  
      # Call the _process_chat_messages method and expect a ValueError
      with pytest.raises(ValueError):
          prompt_builder._process_chat_messages(
              prompt_source, template_variables={}
          )
  
file_path: haystack/test/components/builders/test_dynamic_chat_prompt_builder.py
imports: from typing import List
  import pytest
  from haystack import Pipeline
  from haystack.components.builders import DynamicChatPromptBuilder
  from haystack.dataclasses import ChatMessage
label_desc: Use image node_id 6 for calling the TestDynamicChatPromptBuilder obj's underlying member method code with example usage: obj.test_chat_message_list_with_missing_variables() without return types