fix(resourcegroup/runaway): enhance failpoint control and logging for runaway GC (#64286)

[ti-chi-bot]

This is an automated cherry-pick of #64286

What problem does this PR solve?

Issue Number: ref #41316.

Problem Summary:

Test TestResourceGroupRunawayExceedTiDBSide is flaky due to the short GC interval.

What changed and how does it work?

Updated FastRunawayGC failpoint to accept integer values instead of boolean, enabling finer control of GC intervals in tests.

Check List

Tests

• Unit test
• Integration test

Release note

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

None

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-8.5@7c3720b). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #65506   +/-   ##
================================================
  Coverage               ?   71.2265%           
================================================
  Files                  ?       1681           
  Lines                  ?     469599           
  Branches               ?          0           
================================================
  Hits                   ?     334479           
  Misses                 ?     113322           
  Partials               ?      21798           

Flag	Coverage Δ
integration	46.8153% <12.5000%> (?)
unit	64.9349% <87.5000%> (?)

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

Components	Coverage Δ
dumpling	52.9278% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	40.4679% <0.0000%> (?)

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[EmmaDuDu]

/retest

[ti-chi-bot]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-12 05:40:26.189046021 +0000 UTC m=+249670.250910919: ☑️ agreed by JmPotato.
• 2026-01-12 08:19:33.25583961 +0000 UTC m=+259217.317704519: ☑️ agreed by okJiang.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bufferflies, JmPotato, lhy1024, okJiang

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Needs approval from an approver in each of these files:

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[ti-chi-bot]

@lhy1024: adding LGTM is restricted to approvers and reviewers in OWNERS files.

Details

In response to this:

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