test: add unit tests for handler-base

- tests/conftest.py: Pytest fixtures and configuration
- tests/unit/test_config.py: Settings tests
- tests/unit/test_nats_client.py: NATS client tests
- tests/unit/test_health.py: Health server tests
- tests/unit/test_clients.py: Service client tests
- pytest.ini: Pytest configuration

pytest.ini

@@ -0,0 +1,7 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+asyncio_mode = auto
+addopts = -v --tb=short