Add longhorn disk check before upgrade

During Longhorn upgrades to version 1.9, we observed intermittent failures in CI/CD pipelines. While some upgrade runs succeed, others fail with errors related to the Longhorn admission webhook, for example:

InternalError): Internal error occurred: failed calling webhook "validator.longhorn.io": failed to call webhook: Post "https://longhorn-admission-webhook.longhorn-system.svc:9502/v1/webhook/validation?timeout=10s": no endpoints available for service "longhorn-admission-webhook"
2025/08/12 04:37:21.431101 Kustomization/flux-system state changed: Progressing - Reconciliation in progress
2025/08/12 04:37:22.417396 Kustomization/flux-system state changed: ReconciliationFailed - PersistentVolumeClaim/flux-system/flux-sources-pvc dry-run failed (InternalError): Internal error occurred: failed calling webhook "validator.longhorn.io": failed to call webhook: Post "https://longhorn-admission-webhook.longhorn-system.svc:9502/v1/webhook/validation?timeout=10s": no endpoints available for service "longhorn-admission-webhook"
2025/08/12 04:37:54.263438 Command timeout exceeded
Timed-out waiting for the following resources to be ready:
IDENTIFIER                                                            STATUS     REASON                   MESSAGE
HelmRelease/sylva-system/longhorn                                     Failed                              Failed to upgrade after 4 attempt(s)
├┄╴DaemonSet/longhorn-system/longhorn-manager                         InProgress                          Available: 0/4
┆  ├┄╴Pod/longhorn-system/longhorn-manager-svg4r                      Failed                              Containers in CrashLoop state: longhorn-manager

Logs from longhorn-manager show that disks are still syncing:

Rejected operation: Request (user: system:serviceaccount:longhorn-system:longhorn-service-account, longhorn.io/v1beta2, Kind=Node, namespace: longhorn-system, name: mgmt-1978883962-rke2-capm3-virt-management-cp-2, operation: UPDATE) error="spec and status of disks on node ... are being syncing and please retry later."

While disk syncing is in progress, the Longhorn admission webhook rejects any CRD changes involving those nodes/disks.

This leads to transient errors in upgrade jobs, where some nodes may appear missing or cause upgrade attempts to fail.