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_test.go

@@ -0,0 +1,46 @@
+package agent
+
+import (
+	"testing"
+
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+func readyPod(deletionTimestamp *metav1.Time) *corev1.Pod {
+	return &corev1.Pod{
+		ObjectMeta: metav1.ObjectMeta{DeletionTimestamp: deletionTimestamp},
+		Status: corev1.PodStatus{
+			Conditions: []corev1.PodCondition{
+				{Type: corev1.PodReady, Status: corev1.ConditionTrue},
+			},
+		},
+	}
+}
+
+func TestPodAnycastEligible(t *testing.T) {
+	now := metav1.Now()
+	cases := []struct {
+		name string
+		pod  *corev1.Pod
+		want bool
+	}{
+		{"ready, not deleting", readyPod(nil), true},
+		{"ready, but deleting", readyPod(&now), false},
+		{
+			"not ready, not deleting",
+			&corev1.Pod{Status: corev1.PodStatus{Conditions: []corev1.PodCondition{
+				{Type: corev1.PodReady, Status: corev1.ConditionFalse},
+			}}},
+			false,
+		},
+		{"no conditions, not deleting", &corev1.Pod{}, false},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := podAnycastEligible(c.pod); got != c.want {
+				t.Fatalf("got %v want %v", got, c.want)
+			}
+		})
+	}
+}