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))

tests/test_import_isolation.py

@@ -0,0 +1,37 @@
+"""protocol/ and ocf/ must be vendorable on their own — no dependency
+on mqtt_demo/. This copies just those two directories into an empty
+temp dir and imports every module in them there, so a stray
+`from mqtt_demo... import ...` fails loudly instead of silently
+passing because mqtt_demo/ happens to also be on sys.path in-repo."""
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+
+
+def test_protocol_and_ocf_import_without_mqtt_demo_present(tmp_path):
+    for pkg in ('protocol', 'ocf'):
+        shutil.copytree(REPO_ROOT / pkg, tmp_path / pkg)
+
+    import_lines = [
+        "import protocol.coap",
+        "import protocol.dtls_session",
+        "import ocf.state_cache",
+        "import ocf.poll_scheduler",
+        "import ocf.keepalive",
+        "import ocf.observe_refresh",
+    ]
+    script = "\n".join(import_lines) + "\nprint('OK')\n"
+
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        cwd=str(tmp_path),
+        capture_output=True, text=True,
+    )
+    assert result.returncode == 0, (
+        f"protocol/ocf failed to import without mqtt_demo/ present:\n"
+        f"stdout: {result.stdout}\nstderr: {result.stderr}"
+    )
+    assert "OK" in result.stdout