fix(drone-airspace): refuse empty or incompletely paged FAA snapshots

An upstream answer whose zones all fail validation normalized to an empty
list, and paging that stopped at the page limit while the service still
flagged more records was accepted as complete. Both were cached as a fresh
snapshot for an hour and replaced the last good one. Treat them as failed
refreshes so stale-on-error keeps serving the previous snapshot.

docs/CURRENT-STATE.md

@@ -4100,7 +4100,9 @@ Fire Perimeters uses capped, timed server reads with stale-on-error caching and
 Prohibited & Restricted Airspace is off by default (share-link token `8`) and sits in
 Infrastructure. The proxy pages `TYPE_CODE IN ('P','R')` from the FAA SUA
 FeatureServer, caps the body, and reuses a one-hour snapshot, serving the last
-good payload as stale when a refresh fails. Client-supplied URLs are rejected.
+good payload as stale when a refresh fails. An answer with no valid zone, or
+paging that ends while the service still flags more records, counts as a
+failed refresh. Client-supplied URLs are rejected.
 Polygons keep holes; pick ownership is deliberately unregistered so sibling
 cards still work over a zone. The row legend and selected card state that the
 layer is informational only and not a substitute for B4UFLY / LAANC.