server: check connection is available in SQLKiller (#60685)

[bb7133]

This is a cherry-pick of #60685, and the conflicts are resolved by:

• pkg/server/conn_stmt.go: kept the new SQLKiller connection‑alive hook (IsConnectionAlive.Store(&fn) with defer reset) inserted before executing the prepared statement, so the checker uses bufReadConn.IsAlive() during plan‑cache prepared statement execution.
• pkg/server/tests/commontest/tidb_test.go: preserved the existing audit plugin tests (TestAuditPluginInfoForStarting, TestAuditPluginRetrying) from the release branch and appended the new TestIssue57531 test from the cherry‑picked commit, so both sets of tests remain.

---

What problem does this PR solve?

Issue Number: close #57531

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• 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

• None

Documentation

• None

Release note

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

  - Fix the issue that TiDB can't close connection immediately after killing client process.

[codecov]

Codecov Report

❌ Patch coverage is 94.91525% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-8.5-20260116-v8.5.5@ee48f0b). Learn more about missing BASE report.

Additional details and impacted files

@@                       Coverage Diff                        @@
##             release-8.5-20260116-v8.5.5     #65862   +/-   ##
================================================================
  Coverage                               ?   72.5168%           
================================================================
  Files                                  ?       1683           
  Lines                                  ?     469473           
  Branches                               ?          0           
================================================================
  Hits                                   ?     340447           
  Misses                                 ?     106363           
  Partials                               ?      22663           

Flag	Coverage Δ
integration	49.6510% <58.6206%> (?)
unit	64.9399% <94.9152%> (?)

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	55.4929% <0.0000%> (?)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[EmmaDuDu]

/retest

[YangKeao]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Defined2014, YangKeao

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [Defined2014,YangKeao]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-29 09:40:06.995594017 +0000 UTC m=+1264434.609550863: ☑️ agreed by Defined2014.
• 2026-01-29 09:44:39.87389047 +0000 UTC m=+1264707.487847325: ☑️ agreed by YangKeao.