fix(dataprotection): surface terminating namespace cleanup blocker

[weicao]

What problem does this PR solve?

When a Backup with deletionPolicy: Delete is deleted after its namespace has entered Terminating, DataProtection cannot create the worker resources needed to delete the backup files. The controller intentionally retains the finalizer and retries, but the Backup API has no durable indication of why deletion is blocked; the reason is visible only in controller logs.

Fixes #10694.

What is changed and how does it work?

• return a dedicated internal error when the backup namespace is Terminating
• persist that exact cleanup-order blocker in Backup.status.failureReason
• avoid duplicate status patches for the same reason while continuing to return the original error so reconciliation keeps retrying
• leave other Deleting/Unknown errors, finalizer behavior, worker-resource creation, and deletion semantics unchanged
• do not emit a namespaced Event because Event creation is not reliable after namespace termination begins

Tests

• go test ./controllers/dataprotection -run 'TestDeleteBackupFiles(RecordsTerminatingNamespaceBlockerWithoutStoppingRetry|DoesNotRecordOrdinaryDeletingError|ReturnsStatusPatchError)$' -count=3
• KUBEBUILDER_ASSETS=... go test ./controllers/dataprotection -run TestAPIs -ginkgo.focus 'should not create worker resources when backup namespace is terminating' -count=1
• KUBEBUILDER_ASSETS=... go test ./controllers/dataprotection -count=1
• KUBEBUILDER_ASSETS=... go test ./pkg/dataprotection/backup -count=1
• go vet ./controllers/dataprotection
• make test-go-generate

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[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.00%. Comparing base (116b639) to head (828b093).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
controllers/dataprotection/backup_controller.go	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10693      +/-   ##
==========================================
+ Coverage   63.98%   64.00%   +0.02%     
==========================================
  Files         520      520              
  Lines       62995    63012      +17     
==========================================
+ Hits        40305    40332      +27     
+ Misses      19101    19080      -21     
- Partials     3589     3600      +11     

Flag	Coverage Δ
unittests	64.00% <89.47%> (+0.02%)	⬆️

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