test: verify protocol/ + ocf/ import in isolation from mqtt_demo/

- Add test_import_isolation.py: isolation test that copies protocol/ and ocf/
  to a temp directory without mqtt_demo/ and verifies all modules import
- Add conftest.py: pytest configuration to add repo root to sys.path,
  enabling tests to import protocol/ and ocf/ packages

tests/conftest.py

@@ -0,0 +1,7 @@
+import sys
+from pathlib import Path
+
+# Ensure repo root is on sys.path so that protocol/ and ocf/ can be imported
+repo_root = Path(__file__).resolve().parent.parent
+if str(repo_root) not in sys.path:
+    sys.path.insert(0, str(repo_root))