controlplane/controller: fix flex-algo node-segment not removed on rollback

When flex-algo is disabled after being enabled, the controller was not
emitting 'no node-segment ipv4 index X flex-algo TOPOLOGY' cleanup lines
on Loopback255. Root cause: FlexAlgoNodeSegments was only populated when
flex-algo was enabled, so the template had nothing to iterate over during
rollback.

Fix: populate FlexAlgoNodeSegments whenever a features config is loaded
(not just when enabled), so the template can emit cleanup lines even
when flex-algo is being disabled.

Author: ben-malbeclabs

controlplane/controller/internal/controller/fixtures/base.config.flex-algo-disabled.txt

@@ -79,6 +79,12 @@ interface Ethernet1/1
    no traffic-engineering administrative-group
    no traffic-engineering
 !
+interface Loopback255
+   ip address 14.14.14.14/32
+   node-segment ipv4 index 100
+   no node-segment ipv4 index 200 flex-algo unicast-default
+   isis enable 1
+!
 interface Loopback1000
    description RP Address
    ip address 10.0.0.0/32

controlplane/controller/internal/controller/render_test.go

@@ -907,6 +907,16 @@ func TestRenderConfig(t *testing.T) {
 							LinkStatus:     serviceability.LinkStatusActivated,
 							LinkTopologies: []string{"unicast-default"},
 						},
+						{
+							Name:           "Loopback255",
+							Ip:             netip.MustParsePrefix("14.14.14.14/32"),
+							NodeSegmentIdx: 100,
+							InterfaceType:  InterfaceTypeLoopback,
+							LoopbackType:   LoopbackTypeVpnv4,
+							FlexAlgoNodeSegments: []FlexAlgoNodeSegmentModel{
+								{NodeSegmentIdx: 200, TopologyName: "unicast-default"},
+							},
+						},
 					},
 					Tunnels: []*Tunnel{
 						{

controlplane/controller/internal/controller/server.go

@@ -426,8 +426,10 @@ func (c *Controller) updateStateCache(ctx context.Context) error {
 			linkMetrics.WithLabelValues(device.Code, iface.Name, d.PubKey).Set(float64(d.Interfaces[i].Metric))
 		}
 
-		// Populate flex-algo node-segment data for VPNv4 loopback interfaces
-		if c.featuresConfig != nil && c.featuresConfig.Features.FlexAlgo.Enabled {
+		// Populate flex-algo node-segment data for VPNv4 loopback interfaces.
+		// Populated whenever a features config is loaded (not just when enabled) so that
+		// the template can emit cleanup ("no node-segment") lines on rollback.
+		if c.featuresConfig != nil {
 			for i, intf := range d.Interfaces {
 				if !intf.IsVpnv4Loopback() {
 					continue