Say a read error of a silent rest, and keep the node-engine one at a time in a kept step (hq ADR 0353)

An empty answer on a read error would clear release-silent unseen.

cmd/mesh-controller/gate.go

@@ -1444,7 +1444,11 @@ func probeGates(ctx context.Context, d *doctor) ([]conditions.Observation, error
 	// A release held after a failed one, while builds still wait for a gate (ADR 0236).
 	out = append(out, backlogObservation()...)
 	// And a rest that went silent (novox/hq ADR 0353 §4).
-	out = append(out, silentObservation(ctx, d.open.inventory)...)
+	silent, err := silentObservation(ctx, d.open.inventory)
+	if err != nil {
+		return nil, err
+	}
+	out = append(out, silent...)
 	return sortedFound(dedupeObservations(out)), nil
 }