Add Testing Framework

`pytest` is already included as a development-specific dependency.

- Implement unit tests with mock responses
- Add integration tests for end-to-end validation

Start by inspecting the Python client’s public API and current pytest development dependency to identify the request and endpoint entry points. Add unit coverage using mocked responses and integration coverage for end-to-end validation; done means both suites run successfully and exercise the client’s API behavior.

Written by the indexing model from the issue text.