anycast: drop pods from nexthop set on DeletionTimestamp

Previously the AnycastReconciler kept a pod in the nexthop set as long as
its PodReady condition was True. During a rolling restart that produces a
window after kubelet has accepted SIGTERM (DeletionTimestamp set, pod
still Ready until probes observe shutdown) where BGP still advertises a
path through the dying pod's veth — in-flight requests get RST'd when
the container actually exits.

Fix: introduce podAnycastEligible(pod) = !DeletionTimestamp && Ready,
swap it in at the AnycastReconciler's isReady callback, and fire the
ready-change callback when DeletionTimestamp transitions (the informer
UpdateFunc previously only fired on Ready transitions).

Result: as soon as the apiserver marks a pod for deletion, the
reconciler withdraws the local nexthop and BIRD reannounces the route
without it. Sibling replicas absorb traffic before the pod's
terminationGracePeriod elapses.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

pkg/agent/podinfo.go

@@ -28,6 +28,16 @@ func podReady(pod *corev1.Pod) bool {
 	return false
 }
 
+// podAnycastEligible reports whether a pod should contribute its IP as a
+// nexthop for its anycast IPs. A pod is eligible when it is Ready AND not
+// being deleted. Once the apiserver sets DeletionTimestamp, kubelet has
+// started teardown — kube-proxy will keep routing for terminationGracePeriod
+// but the pod is on the way out; we should withdraw the nexthop immediately
+// so BGP shifts traffic to a sibling before the pod actually exits.
+func podAnycastEligible(pod *corev1.Pod) bool {
+	return pod.DeletionTimestamp == nil && podReady(pod)
+}
+
 // PodCache exposes a Get(ns, name) lookup against a node-scoped Pod
 // informer. ADD/DEL handlers consult it to read annotations + labels for
 // IPAM and (later) NetworkPolicy. Callers can subscribe to Ready
@@ -58,7 +68,7 @@ func StartPodInformer(ctx context.Context, cfg *rest.Config, node string, logger
 
 	_, _ = inf.AddEventHandler(cache.ResourceEventHandlerFuncs{
 		AddFunc: func(obj interface{}) {
-			if pod, ok := obj.(*corev1.Pod); ok && podReady(pod) {
+			if pod, ok := obj.(*corev1.Pod); ok && podAnycastEligible(pod) {
 				pc.fireReady()
 			}
 		},
@@ -68,7 +78,10 @@ func StartPodInformer(ctx context.Context, cfg *rest.Config, node string, logger
 			if oldP == nil || newP == nil {
 				return
 			}
-			if podReady(oldP) != podReady(newP) {
+			// Fire on Ready transition OR DeletionTimestamp transition.
+			// The latter catches "pod was Ready, now being deleted" so the
+			// reconciler withdraws the nexthop before the pod actually exits.
+			if podAnycastEligible(oldP) != podAnycastEligible(newP) {
 				pc.fireReady()
 			}
 		},