roachtest: deflake online-restore recovery

[kev-cao]

The OR recovery roachtest was failing due to the schemachange workload truncating a table and the roachtest only deleting from the full backup. In certain scenarios, the SST deleted from the full backup would end up being skipped in the restore due to the span it covered never being covered after truncation.

This commit teaches OR to instead delete an SST from every layer of the backup collection, which should make the test resilient to truncation.

Fixes: #159503

Release note: None

[cockroach-teamcity]

This change is 

[github-actions]

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[Copilot]

Pull request overview

This PR addresses a flaky roachtest in the online-restore recovery functionality. The test was failing when the schemachange workload truncated tables, because the test only deleted SSTs from the full backup layer. After truncation, the deleted SST could be skipped during restore if its span was no longer covered.

Key Changes:

• Renamed deleteUserTableSST to deleteSSTFromBackupLayers to reflect the new behavior
• Modified the function to delete one SST from each layer (directory) of the backup collection instead of just one SST from the full backup
• Updated SQL query to use a CTE with regex-based directory extraction and window functions to select one SST per layer

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
pkg/cmd/roachtest/tests/mixed_version_backup.go	Refactored SST deletion logic to delete one SST from each backup layer, making the test resilient to table truncation
pkg/cmd/roachtest/tests/backup_restore_roundtrip.go	Updated function call to use the renamed deleteSSTFromBackupLayers method

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[msbutler]

Looks good! if you haven't already done so, maybe run the test 5 times to make sure there are no further related flakes?

[kev-cao]

@msbutler Yup, already ran it a couple of times! Although to be honest the flakes for these tests are so infrequent, I think they'd have to run for quite a while. We usually go a couple months before a flake on this test.

[kev-cao]

TFTR!

bors r=msbutler

[craig]

Build succeeded:

• local_roachtest_fips
• linux_amd64_build
• unit_tests
• linux_amd64_fips_build
• docker_image_amd64
• lint
• acceptance
• check_generated_code
• examples_orms
• local_roachtest

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #159503: branch-release-25.3, branch-release-25.4.

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