fix: backup deletion silently succeeds when tarball download fails

[priyansh17]

Enhance backup deletion logic to handle tarball download failures clean up associated CSI VolumeSnapshotContents

Please add a summary of your change

When downloadToTempFile fails during backup deletion (e.g., transient Azure credential throttling HTTP 429), the error was only logged; never recorded in errs. The deletion proceeded as successful: Backup CR deleted, DeleteBackupRequest marked Processed with zero errors, and VolumeSnapshotContent objects (+ underlying cloud snapshots) permanently leaked with no user-visible failure.

• Added isTarballNotFoundError() helper that distinguishes permanent not-found errors (HTTP 404, BlobNotFound, NoSuchKey, ObjectNotExist, etc.) from transient errors (throttling, auth failures, network issues).
• For transient errors: append the error to errs so:
  • DeleteBackupRequest.Status.Errors captures the failure reason
  • Backup CR is not deleted (len(errs) == 0 gate)
  • RegisterBackupDeletionFailed fires instead of success
  • Customer can retry after the transient issue resolves

Does your change fix a particular issue?

Fixes #9685

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.97%. Comparing base (15db9d2) to head (0dc8162).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/controller/backup_deletion_controller.go	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9693      +/-   ##
==========================================
+ Coverage   60.96%   60.97%   +0.01%     
==========================================
  Files         384      384              
  Lines       36595    36608      +13     
==========================================
+ Hits        22310    22323      +13     
- Misses      12676    12677       +1     
+ Partials     1609     1608       -1     

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