test(vector): address PR #873 round-3 nits

- Simplify the OCS status guard to `if status and status != "ok"` — falsy
  (missing/None/"") is tolerated more naturally than the explicit tuple.
- Add `test_value_error_from_ocs_failure_returns_none`, covering the
  OCS-failure ValueError flowing through `_get_enabled_apps_or_none` to the
  scan-all fallback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/__init__.py

@@ -225,7 +225,7 @@ class NextcloudClient:
         # cycle on an empty ``data``. Tolerate a missing/empty meta (our own
         # mocks, and any envelope that omits it).
         status = (ocs.get("meta") or {}).get("status")
-        if status not in ("ok", None, ""):
+        if status and status != "ok":  # falsy (missing/None/"") tolerated
             raise ValueError(f"OCS navigation returned status={status!r}")
         entries = ocs.get("data") or []
         enabled: set[str] = set()