WIP: statistics: persist stats delta init time

[0xPoe]

What problem does this PR solve?

Issue Number: close #65426

Problem Summary:

• Small post-ANALYZE modifications could stay unflushed because delta InitTime wasn't persisted, so the time-based dump never triggered.

What changed and how does it work?

• Persist TableDelta.InitTime across sweeps and write it back when deciding to skip a dump.
• Add a regression test and test-only accessors for the dump duration threshold.
• Update Bazel files via make bazel_prepare for the new test file.

Check List

Tests

• Unit test
  • go test ./pkg/statistics/handle/usage -run TestDumpStatsDeltaPersistsInitTime --tags=intest
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix stats delta flushing after small modifications by persisting delta init time for time-based dumps.

[ti-chi-bot]

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Details

Needs approval from an approver in each of these files:

• pkg/statistics/OWNERS

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[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.8345%. Comparing base (8333420) to head (b8f9f66).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65586        +/-   ##
================================================
- Coverage   77.8457%   77.8345%   -0.0113%     
================================================
  Files          1978       1902        -76     
  Lines        542181     529874     -12307     
================================================
- Hits         422065     412425      -9640     
+ Misses       118457     116973      -1484     
+ Partials       1659        476      -1183     

Flag	Coverage Δ
integration	44.2513% <0.0000%> (-3.9385%)	⬇️
unit	76.1198% <90.0000%> (-0.3394%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	56.7974% <ø> (ø)
parser	∅ <ø> (∅)
br	48.7901% <ø> (-12.3524%)	⬇️

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[ti-chi-bot]

@0xPoe: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	b8f9f66	link	true	/test unit-test

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