code: def test_single_chat_message(self):
      prompt_builder = DynamicChatPromptBuilder(
          runtime_variables=["documents"]
      )
      prompt_source = [
          ChatMessage.from_user(content="Hello, {{ who }}!")
      ]
      template_variables = {"who": "World"}
  
      result = prompt_builder._process_chat_messages(
          prompt_source, template_variables
      )
  
      assert result == [ChatMessage.from_user(content="Hello, World!")]
  
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 3 for calling the TestDynamicChatPromptBuilder obj's underlying member method code with example usage: obj.test_single_chat_message() without return types