fix(dataprotection): label backup workloads for cleanup

[weicao]

Closes #10675

What problem does this solve?

Backup-owned Jobs and continuous backup StatefulSets could inherit a non-DataProtection app.kubernetes.io/managed-by label from the parent Backup. DataProtection watch and cleanup selectors require kubeblocks-dataprotection, so those workloads could be missed.

What is changed?

• Force BuildBackupWorkloadLabels to set managed-by=kubeblocks-dataprotection after copying parent labels.
• Keep the parent Backup labels unchanged.
• Keep existing watch and cleanup selectors narrow.
• Cover helper output, parent non-mutation, nil labels, Action Job, BackupData Job, and continuous StatefulSet metadata/template labels.

Existing workloads with the historical wrong label are not migrated by this change.

Validation

• Focused tests N=3
• Focused race test
• Full pkg/dataprotection/backup and controllers/dataprotection tests
• go vet, lint, generated-file checks, and full make check-diff

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.34%. Comparing base (12611aa) to head (40b7b2a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10672      +/-   ##
==========================================
+ Coverage   63.97%   64.34%   +0.37%     
==========================================
  Files         520      520              
  Lines       62887    63298     +411     
==========================================
+ Hits        40229    40730     +501     
+ Misses      19071    19000      -71     
+ Partials     3587     3568      -19     

Flag	Coverage Δ
unittests	64.34% <100.00%> (+0.37%)	⬆️

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