test: isolate VPN DNS settling tests from host adapters

cmd/cli/vpn_dns.go

@@ -52,6 +52,9 @@ type vpnDNSManager struct {
 	// state for one guarded refresh cycle because Windows can briefly report an
 	// intermediate empty adapter/DNS state after sleep/wake or reconnect.
 	retainedAfterEmptyDiscovery bool
+	// discoverVPNDNS is injected for tests so Refresh does not depend on the
+	// runner host's real VPN/virtual adapter state.
+	discoverVPNDNS func(context.Context) []ctrld.VPNDNSConfig
 	// Called when VPN DNS server list changes, to update intercept exemptions.
 	onServersChanged vpnDNSExemptFunc
 }
@@ -63,6 +66,7 @@ func newVPNDNSManager(logger *atomic.Pointer[ctrld.Logger], exemptFunc vpnDNSExe
 	return &vpnDNSManager{
 		routes:           make(map[string][]string),
 		logger:           logger,
+		discoverVPNDNS:   ctrld.DiscoverVPNDNS,
 		onServersChanged: exemptFunc,
 	}
 }
@@ -74,7 +78,11 @@ func (m *vpnDNSManager) Refresh(ctx context.Context, guardAgainstNoNameservers .
 	guardedRefresh := len(guardAgainstNoNameservers) > 0 && guardAgainstNoNameservers[0]
 
 	ctrld.Log(ctx, logger.Debug(), "Refreshing VPN DNS configurations")
-	configs := ctrld.DiscoverVPNDNS(ctx)
+	discoverVPNDNS := m.discoverVPNDNS
+	if discoverVPNDNS == nil {
+		discoverVPNDNS = ctrld.DiscoverVPNDNS
+	}
+	configs := discoverVPNDNS(ctx)
 
 	// Detect exit mode: if the default route goes through a VPN DNS interface,
 	// the VPN is routing ALL traffic (exit node / full tunnel). This is more

cmd/cli/vpn_dns_test.go

@@ -21,6 +21,7 @@ func TestVPNDNSRefreshRetainsStateForOneGuardedEmptyDiscovery(t *testing.T) {
 		gotExemptions = exemptions
 		return nil
 	})
+	m.discoverVPNDNS = func(context.Context) []ctrld.VPNDNSConfig { return nil }
 	m.configs = []ctrld.VPNDNSConfig{{
 		InterfaceName: "Ethernet 6",
 		Servers:       []string{"10.25.37.21", "10.25.37.22"},
@@ -47,6 +48,7 @@ func TestVPNDNSRefreshClearsOnSecondGuardedEmptyDiscovery(t *testing.T) {
 		gotExemptions = exemptions
 		return nil
 	})
+	m.discoverVPNDNS = func(context.Context) []ctrld.VPNDNSConfig { return nil }
 	m.configs = []ctrld.VPNDNSConfig{{
 		InterfaceName: "Ethernet 6",
 		Servers:       []string{"10.25.37.21"},