chore: placeholder test so pytest does not exit 5 on empty scaffold

Final piece of #9: with no tests at all, pytest exits with code 5 (no tests
collected) and CI's Unit tests + Coverage jobs fail. test_placeholder makes
the suite collectable; it is replaced by real tests in #17/#18/#19.

Author: constk

tests/test_placeholder.py

@@ -0,0 +1,9 @@
+"""Placeholder so pytest does not exit with code 5 (no tests collected) on the
+empty scaffold. Real tests land alongside #17 / #18 / #19; this file is
+removed once those tests exist and exercise the suite."""
+
+from __future__ import annotations
+
+
+def test_placeholder() -> None:
+    assert 1 + 1 == 2