fix: allow set empty for backupVolumeSnapshotClassName in Setting csi-driver-config (backport #1282)

[mergify]

Summary

The backupVolumeSnapshotClassName should be grayed out directly for third-party storage.

PR Checklists

• Do we need to backport this PR change to the Harvester Dashboard?
  • Yes, the relevant PR is at:
• Are backend engineers aware of UI changes?
  • Yes, the backend owner is: @WebberHuang1118

Related Issue

[BUG][GUI] Should Allow set empty for backupVolumeSnapshotClassName in Setting csi-driver-config #7737

Test screenshot/video

• Navigate to Advanced → Settings → edit csi-driver-config
• Click Add, and the Backup Volume Snapshot Class Name field should be disabled
• Select the appropriate option, save and ensure it saves correctly

Extra technical notes summary

N/A

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This is an automatic backport of pull request #1282 done by [Mergify](https://mergify.com).