test: add unit and integration test suite for the new SDK

- 268 unit tests covering all sync and async client methods
- 196 integration tests against a real sandbox API
- Mock-based tests using respx for sync and async HTTP mocking
- Paging, error handling, and edge case coverage (400/401/403/404/500/418)
- Integration test utilities and test data files
- .env.example with sandbox environment variables

tests/integration/testdata/test-attachment.txt

@@ -0,0 +1 @@
+This is a test attachment.