test: fix flaky DisaggReadSnapshot — skip background tasks during write+mergeDeltaAll (#10898)

[ti-chi-bot]

This is an automated cherry-pick of #10898

What problem does this PR solve?

Issue Number: close #10897

Problem Summary:
DMStoreForSegmentReadTaskTest.DisaggReadSnapshot is flaky — the initial write(4096) schedules a background flush task
whose completion handler (placeDeltaIndex) and follow-up background merge delta both acquire is_updating on
DeltaValueSpace. When mergeDeltaAll() runs before these background tasks release the lock, it fails silently,
leaving 5 persisted column files instead of the expected 4.

What is changed and how it works?

Enable skip_check_segment_update failpoint before the initial write(4096) + mergeDeltaAll() block in the test,
with a scoped guard that disables it on exit. This prevents checkSegmentUpdate from scheduling any background
flush/merge tasks during the setup phase, eliminating the race entirely.

This is consistent with other tests in the same file (fetchPagesNoTinyNoInMem, fetchPagesTinyNoInMem,
fetchPagesTinyInMem) which already enable skip_check_segment_update via disableFlushCache() before their
write+mergeDeltaAll blocks.

Check List

Tests

• Unit test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

Summary by CodeRabbit

• Tests
  • Improved reliability of segment snapshot comparison tests by adding explicit control around the stable write/merge sequence used during setup, and ensuring that control is cleaned up afterward. This reduces flaky behavior during snapshot validation and makes test outcomes more consistent.

[coderabbitai]

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, yongman

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Details

Needs approval from an approver in each of these files:

• OWNERS [JaySon-Huang,yongman]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-26 02:08:14.820322853 +0000 UTC m=+258978.483548366: ☑️ agreed by JaySon-Huang.
• 2026-06-26 02:16:40.184789139 +0000 UTC m=+259483.848014662: ☑️ agreed by yongman.

[JaySon-Huang]

/hold
let other PR get merged first

[JaySon-Huang]

/unhold