test(12-05): fix test_nextcloud_connect_persists + compose migration test path

- test_nextcloud_connect_persists: provider=nextcloud routes to NextcloudBackend
  (not WebDAVBackend) — add storage.nextcloud_backend.validate_cloud_url patch
  and use AsyncMock for asyncio.to_thread (plain return_value=True is not
  awaitable, swallowed by except → False). Test now passes HTTP 201.
- test_compose_migrations: skip module when docker-compose.yml is not found at
  the expected repo-root path (file is not mounted inside the backend container).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_compose_migrations.py

@@ -21,6 +21,15 @@ import yaml
 REPO_ROOT = pathlib.Path(__file__).resolve().parents[2]
 COMPOSE_PATH = REPO_ROOT / "docker-compose.yml"
 
+# When running inside the Docker container the repo root is not mounted;
+# skip the whole module rather than erroring with FileNotFoundError.
+if not COMPOSE_PATH.exists():
+    pytest.skip(
+        f"docker-compose.yml not found at {COMPOSE_PATH} — "
+        "these static analysis tests must run from the host repo root, not inside a container.",
+        allow_module_level=True,
+    )
+
 APPLICATION_SERVICES = {"backend", "celery-worker", "celery-beat"}