fix: handle failed storage VM snapshot cleanup in StorageVMSnapshotStrategy

[waterWang]

Fix: Handle failed storage VM snapshot cleanup in StorageVMSnapshotStrategy

Problem

When a KVM disk-only VM snapshot (created via StorageVMSnapshotStrategy) fails during creation — for example, when the QEMU guest agent is not connected and the freeze operation fails — the snapshot enters the Error state. The STORAGE_SNAPSHOT detail is removed during the creation rollback.

When the user later tries to delete this failed snapshot, StorageVMSnapshotStrategy.canHandle() returns CANT_HANDLE because the STORAGE_SNAPSHOT detail is absent. The deletion falls through to DefaultVMSnapshotStrategy, which sends a DeleteVMSnapshotCommand to the KVM agent. For a stopped VM with RAW/RBD volumes, the libvirt domain does not exist, so the command fails with:

Delete Instance Snapshot failed due to org.libvirt.LibvirtException:
Domain not found: no domain with matching name 'i-2-43-VM'

This is a RAW/RBD variant of issue #11673. PR #11687 fixed a similar case for QCOW2/stopped VMs, but the fallback only handles QCOW2 volumes.

Fix

Two changes in StorageVMSnapshotStrategy:

1. canHandle(VMSnapshot): Allow Error state snapshots (that would otherwise be handled by StorageVMSnapshotStrategy) to skip the STORAGE_SNAPSHOT detail check. Since the detail is removed during the creation rollback, failed snapshots would otherwise be rejected.

2. deleteVMSnapshot override: For Error state storage snapshots, skip the hypervisor DeleteVMSnapshotCommand (which would fail on a stopped VM) and clean up the database record directly via deleteVMSnapshotFromDB. This is safe because the takeVMSnapshot rollback already removed any underlying storage snapshots that were created before the failure.

FSM validation

The Error → Event.ExpungeRequested → Expunging transition is valid (confirmed in VMSnapshot.State).

Testing

• Error state snapshots with KVM + Disk type + VmStorageSnapshotKvm enabled are now handled by StorageVMSnapshotStrategy instead of DefaultVMSnapshotStrategy
• For Error state, deletion bypasses the hypervisor command and cleans up the DB directly
• Ready state snapshots continue to use the inherited deleteVMSnapshot flow (unchanged behavior)

[DaanHoogland]

too high comment/code ratio. please refactor sensible comments to the javadoc of the relevant interfaces and clarify remaining code with using good noun/verbs for identifiers.