Use k8s.io/api well-known label constants instead of hardcoded strings (#10279)

* refactor: use k8s.io/api well-known label constants

Several well-known Kubernetes label strings were hardcoded across the
codebase instead of using the constants already exported by
k8s.io/api/core/v1, which is an existing dependency:

  "kubernetes.io/hostname"        -> corev1api.LabelHostname
  "kubernetes.io/os"              -> corev1api.LabelOSStable
  "topology.kubernetes.io/zone"   -> corev1api.LabelTopologyZone

The local kube.NodeOSLabel and zoneLabel consts, which duplicated the
upstream values verbatim, are now defined in terms of the upstream
constants rather than repeating the literal. Both are kept: NodeOSLabel
is exported and referenced from four packages alongside NodeOSLinux and
NodeOSWindows, which have no upstream equivalent, and zoneLabel sits
beside the deprecated-label fallback it is compared against.

No functional change - every replacement is a constant with an identical
value.

Signed-off-by: Harshit saini <harshitsaini1188@gmail.com>

* Add changelog for #10279

Signed-off-by: Harshit saini <harshitsaini1188@gmail.com>

* Cover the selected-node path in createRestorePod

TestCreateRestorePod only exercised selectedNode == "", so the branch
that pins the restore pod to a node was never executed. Add a case with
a selected node and assert the resulting pod carries the hostname label
in its node selector.

Signed-off-by: Harshit saini <harshitsaini1188@gmail.com>

* Also use constants for the arch and deprecated zone labels

Extends the same replacement to the two remaining well-known labels
raised on the issue:

  "kubernetes.io/arch"                      -> corev1api.LabelArchStable
  "failure-domain.beta.kubernetes.io/zone"  -> corev1api.LabelFailureDomainBetaZone

zoneLabelDeprecated in item_backupper.go was the last local const still
repeating a literal that upstream already exports, so the zone pair now
reads consistently against k8s.io/api. The deprecation note upstream
applies to the label itself, not the constant; Velero reads that label
deliberately as the fallback for PVs created before the topology labels
existed.

Signed-off-by: Harshit saini <harshitsaini1188@gmail.com>

---------

Signed-off-by: Harshit saini <harshitsaini1188@gmail.com>

pkg/controller/restore_finalizer_controller_test.go

@@ -676,11 +676,11 @@ func TestNeedPatch(t *testing.T) {
 		{
 			name: "same label key different values",
 			newPV: builder.ForPersistentVolume("pv1").
-				ObjectMeta(builder.WithLabels("topology.kubernetes.io/zone", "us-west-2a")).
+				ObjectMeta(builder.WithLabels(corev1api.LabelTopologyZone, "us-west-2a")).
 				ReclaimPolicy(corev1api.PersistentVolumeReclaimDelete).Result(),
 			pvInfo: &volume.PVInfo{
 				ReclaimPolicy: string(corev1api.PersistentVolumeReclaimDelete),
-				Labels:        map[string]string{"topology.kubernetes.io/zone": "us-east-1a"},
+				Labels:        map[string]string{corev1api.LabelTopologyZone: "us-east-1a"},
 			},
 			expected: false,
 		},