Fix ListBackups to skip directories without metadata file

[mpryc]

When using object stores with suspended versioning (e.g., MinIO), deleted backup folders persist as empty directories. The S3 API returns these directories, but they lack the velero-backup.json metadata file.

This change adds a check in ListBackups() to verify that the backup metadata file exists before including a backup in the results. Empty directories without metadata are now logged at debug level and skipped.

Suggested-by: @chrisamti

Fixes #8466

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[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@2376b69). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/persistence/object_store.go	42.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

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##             main    #9511   +/-   ##
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  Coverage        ?   60.53%           
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  Files           ?      386           
  Lines           ?    36378           
  Branches        ?        0           
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  Hits            ?    22023           
  Misses          ?    12773           
  Partials        ?     1582           

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[blackpiglet]

What did you expect to happen:

Velero should either work with versioned s3 buckets or not pickup the metadata from the versioned blobs if a file was deleted (ex:mc works correctly with versions and does not list any deleted file that has "active" versions)

From the issue's description, I think Velero can read the velero-backup.json of the backup even if the backup is deleted.

I'm still checking with the issue reporter to make sure that.

If that is true, then the current fix cannot address the original issue.