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/test_ping.py

@@ -0,0 +1,15 @@
+import pytest
+
+pytestmark = pytest.mark.integration
+
+
+class TestCanConnect:
+    @pytest.mark.flaky(reruns=3)
+    def test_sync(self, client):
+        result = client.can_connect()
+        assert result is True
+
+    @pytest.mark.flaky(reruns=3)
+    async def test_async(self, async_client):
+        result = await async_client.can_connect()
+        assert result is True