[Cluster] Refactor: Add FinalizeDeleteCluster for async cluster lifecycle management

[powerkimhub]

Add FinalizeDeleteCluster for Two-Phase Cluster Deletion

• Remove meta information cleanup from DeleteCluster so it only deletes CSP resources
• Add FinalizeDeleteCluster() to delete Spider meta information only when the cluster no longer exists on CSP
• Add Cluster NotFound status for clusters that exist in meta but not on CSP
• Add REST route DELETE /spider/cluster/{Name}/finalize for FinalizeDeleteCluster
• Add Finalize button in AdminWeb cluster.html with mixed selection prevention and NotFound display

Azure Driver - Async Cluster Operations

• Make CreateCluster async by removing SG merge wait, returning immediately with Status: Creating (AWS EKS pattern)
• Make DeleteCluster async by removing PollUntilDone, returning after BeginDelete call
• Change GetCluster ResourceNotFound log level from Error to Info (expected during async provisioning)
• Remove unnecessary DNS Zone + WebAppRouting creation/deletion (not used by CB-Spider, no other CSP driver provides it)

AdminWeb - Fix Zone-less Region Support (e.g., Azure westus)

• Fix fetchZone() in vpc-subnet.html to handle undefined ZoneList with "No Zone Available" fallback
• Fix validateForm() in vpc-subnet.html to skip zone validation when not required
• Remove zone requirement check from Image/Spec search overlays in vm.html
• Fix MC-Insight region dropdown to show all regions with alert when current region is not available
• Fix showError() in cluster.html to split on '. ' instead of '.' to preserve version numbers