Add opt-in live API end-to-end test

- test_e2e_api.py: minimal 2-address (4-element) call to the real Google
  Routes API; reads the real key from env/.env and skips on a placeholder.
- Marked `e2e` and deselected by default via addopts; run with
  `pytest -m e2e`.

pyproject.toml

@@ -28,4 +28,8 @@ build-backend = "hatchling.build"
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
-addopts = "-q"
+# Deselect live API tests by default; run them with `pytest -m e2e`.
+addopts = "-q -m 'not e2e'"
+markers = [
+    "e2e: live tests that call the real Google Routes API (opt-in, costs quota)",
+]