[client] make AllowedIPs swap self-healing on WireGuard errors

Address CodeRabbit review on #6799.

Decrement now decides whether to reprogram based on whether the currently
installed peer still holds references (e.peers[e.active] > 0) instead of
whether the released peer was the active one. If a prior swap's remove/add
failed, e.active was left pointing at a peer with zero references and every
later Decrement returned early, permanently stranding the prefix and leaking
the entry. The active peer is now detached only when e.active != "", and a
failed hand-off is retried on the next Decrement/Increment.

Also clear currentPeerKey unconditionally in the static handler's
RemoveAllowedIPs (matching dynamic/dnsinterceptor) and assert Increment
errors in the tests. Added self-heal tests for the failed remove/add paths.

client/internal/routemanager/static/route.go

@@ -62,9 +62,10 @@ func (r *Route) AddAllowedIPs(peerKey string) error {
 }
 
 func (r *Route) RemoveAllowedIPs() error {
-	if _, err := r.allowedIPsRefcounter.Decrement(r.route.Network, r.currentPeerKey); err != nil {
-		return err
+	var err error
+	if _, decErr := r.allowedIPsRefcounter.Decrement(r.route.Network, r.currentPeerKey); decErr != nil {
+		err = fmt.Errorf("remove allowed IP %s: %w", r.route.Network, decErr)
 	}
 	r.currentPeerKey = ""
-	return nil
+	return err
 }