Fix tests

[brylie]

Related to #19

Fix tests by setting the OPENAI_API_KEY environment variable in the test files.

• tests/test_chat_gpt_client.py

  • Add monkeypatch.setenv("OPENAI_API_KEY", "test_api_key") to the test_get_chat_response_with_history_success function.
  • Add monkeypatch.setenv("OPENAI_API_KEY", "test_api_key") to the test_get_chat_response_with_history_api_error function.
  • Add monkeypatch.setenv("OPENAI_API_KEY", "test_api_key") to the test_get_chat_response_with_history_empty_message function.
  • Add monkeypatch.setenv("OPENAI_API_KEY", "test_api_key") to the test_get_chat_response_with_history_parameterized function.

• tests/test_main.py

  • Add monkeypatch.setenv("OPENAI_API_KEY", "test_api_key") to the mock_services fixture.

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