CAS: backup native copy

[k-morozov]

BACKUP of a table on a CAS to an S3 destination that shares the pool's endpoint failed outright, and the code path that failed would have silently corrupted the backup had it not failed.

BackupWriterS3::copyFileFromDisk asks S3 to copy the source object server-side. That assumes the file is that object, whole, from byte 0. On a CAS disk neither half holds:

• A small per-part file (checksums.txt, count.txt, columns.txt, ...) is an inline manifest entry with no object of its own. getStorageObjects returns a sized placeholder with an empty remote key, deliberately poisoned so a reader that bypasses the read path fails loudly rather than reading someone else's bytes. The copy passed that empty key to CopyObject, so every BACKUP died with Invalid argument - these files exist in every part.

• A blob object is [envelope][payload] and the payload is the file. BlobLocation carries the payload offset, but getStorageObjects returns only key and length and getBlobPath keeps only the key, so the copy started at byte 0 and pulled the envelope into the backup. RESTORE then failed.

  The first failure masked the second: checksums.txt aborted the backup before any blob corruption
  could surface. Fixing only the empty key would have turned a loud failure into backups that report
  success and cannot be restored.

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fixed BACKUP of a table on a CAS to an S3 destination on the same endpoint.

Documentation entry for user-facing changes

Adds docs/en/antalya/cas/operations/backup.md describing how BACKUP/RESTORE behave on a content-addressed disk, and links it from the CAS index and roadmap.

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Unit tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• CAS (content-addressed storage; Antalya only)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [b130c47]