[client] Fix iOS DNS upstream routing for deselected exit nodes (#5803)

- Add GetSelectedClientRoutes() to the route manager that filters through FilterSelectedExitNodes, returning only active routes instead of all management routes              
  - Use GetSelectedClientRoutes() in the DNS route checker so deselected exit nodes' 0.0.0.0/0 no longer matches upstream DNS IPs — this prevented the resolver from switching
  away from the utun-bound socket after exit node deselection                                                                                                                   
  - Initialize iOS DNS server with host DNS fallback addresses (1.1.1.1:53, 1.0.0.1:53) and a permanent root zone handler, matching Android's behavior — without this, unmatched
   DNS queries arriving via the 0.0.0.0/0 tunnel route had no handler and were silently dropped

client/internal/dns/server.go

@@ -187,11 +187,16 @@ func NewDefaultServerIos(
 	ctx context.Context,
 	wgInterface WGIface,
 	iosDnsManager IosDnsManager,
+	hostsDnsList []netip.AddrPort,
 	statusRecorder *peer.Status,
 	disableSys bool,
 ) *DefaultServer {
+	log.Debugf("iOS host dns address list is: %v", hostsDnsList)
 	ds := newDefaultServer(ctx, wgInterface, NewServiceViaMemory(wgInterface), statusRecorder, nil, disableSys)
 	ds.iosDnsManager = iosDnsManager
+	ds.hostsDNSHolder.set(hostsDnsList)
+	ds.permanent = true
+	ds.addHostRootZone()
 	return ds
 }