Ask a declared tool check on the NATS link the node-engine holds

The queue asks a tool through a ToolBus assertion that only OverNATS satisfied; the
link the engine actually holds (natsLink) had no Ask, so every tool check failed as
"no link to the bus is open" while the link was up (hq issue 331). Give natsLink Ask,
assert at build time every optional interface the queue expects of it, and tell a link
that cannot ask apart from no link.

internal/link/queue.go

@@ -530,10 +530,13 @@ func (q *Queue) AskTool(ctx context.Context, module, tool string) (bool, string,
 	q.mu.Lock()
 	bus := q.bus
 	q.mu.Unlock()
-	asker, ok := bus.(ToolBus)
-	if bus == nil || !ok {
+	if bus == nil {
 		return false, "", errors.New("no link to the bus is open")
 	}
+	asker, ok := bus.(ToolBus)
+	if !ok {
+		return false, "", errors.New("the link to the bus open now cannot ask a tool")
+	}
 	reply, err := asker.Ask(ctx, ToolSubject(module, tool, q.Membership.Node), []byte("{}"))
 	if err != nil {
 		return false, "", err