resourcegroup: replace blind sleep with signal-aware wait in acquireTokens (#10252)

[ti-chi-bot]

This is an automated cherry-pick of #10252

What problem does this PR solve?

Issue Number: close #10251.

What is changed and how does it work?

Add a close-and-recreate channel on the `Limiter` that is closed on every `Reconfigure()` call.
The retry loop in `acquireTokens` now selects on this channel instead of blind-sleeping,
waking up immediately when new tokens arrive. The timer serves as a fallback to preserve original
behavior when no `Reconfigure` occurs.

Check List

Tests

• Unit test
• Integration test

Release note

None.

Summary by CodeRabbit

• Bug Fixes

  • Token acquisition retry now uses an interruptible wait so reconfiguration cancels unnecessary delays, improving responsiveness during limiter changes.

• New Features

  • Limiter now signals reconfiguration to promptly wake waiting operations.

• Tests

  • Added coverage for signal-aware wakeups, timer fallback behavior, and waking multiple waiters during reconfiguration.

[ti-chi-bot]

@JmPotato This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
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[JmPotato]

/retest

[JmPotato]

/retest

[codecov]

Codecov Report

❌ Patch coverage is 59.37500% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.85%. Comparing base (9c9a8c8) to head (79eb1f1).

❌ Your patch check has failed because the patch coverage (59.37%) is below the target coverage (74.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@                       Coverage Diff                       @@
##           release-8.5-20260323-v8.5.5   #10505      +/-   ##
===============================================================
- Coverage                        77.98%   77.85%   -0.14%     
===============================================================
  Files                              482      482              
  Lines                            65391    65419      +28     
===============================================================
- Hits                             50997    50932      -65     
- Misses                           10665    10747      +82     
- Partials                          3729     3740      +11     

Flag	Coverage Δ
unittests	77.85% <59.37%> (-0.14%)	⬇️

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