class_docstr: Unit tests for Agent class
code: def test_chat(self, sample_df, config):
      # Create an Agent instance for testing
      agent = Agent(sample_df, config)
      agent._lake.chat = Mock()
      agent._lake.chat.return_value = (
          "United States has the highest gdp"
      )
      # Test the chat function
      response = agent.chat("Which country has the highest gdp?")
      assert agent._lake.chat.called
      assert isinstance(response, str)
      assert response == "United States has the highest gdp"
  
imports: from typing import Optional
  from unittest.mock import Mock
  from pandasai.agent import Agent
  import pandas
  import pytest
  from pandasai.llm.fake import FakeLLM
  from pandasai.prompts.clarification_questions_prompt import ClarificationQuestionPrompt
  from pandasai.prompts.explain_prompt import ExplainPrompt
  from pandasai.smart_datalake import SmartDatalake
label_desc: Use image node_id 6 for calling the TestAgent obj's underlying member method code with example usage: obj.test_chat(sample_df, config) without return types