test: stabilize TestAuditPluginRetrying

[tangenta]

What problem does this PR solve?

Issue Number: close #65222

Problem Summary:

• require.Eventually with the predicate "we observed more audit events than issued statements" checks len(testResults) > concurrency, which only becomes true if at least one statement was retried (extra audit events).
• Before the change, testResults was a shared slice appended by many server goroutines (audit callbacks) while the test goroutine concurrently reset/read it, with no synchronization. Under load this can lose/corrupt entries, so len(testResults) can stay <= concurrency even if retries happened.

What changed and how does it work?

• The change makes testResults thread-safe (mutex + snapshot helpers), and also removes DB usage patterns that make the loop less reliable: it switches from Conn().QueryContext() (no Rows/Conn close, error asserts in goroutines) to db.ExecContext() with a sized pool (SetMaxOpenConns/SetMaxIdleConns) and collects errors back in the main goroutine.

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

• 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.

None

[tiprow]

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[tangenta]

/check-issue-triage-complete

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.6614%. Comparing base (69a1b2c) to head (6f3628b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65523        +/-   ##
================================================
- Coverage   77.8448%   77.6614%   -0.1835%     
================================================
  Files          1971       1895        -76     
  Lines        540836     528530     -12306     
================================================
- Hits         421013     410464     -10549     
+ Misses       118164     118060       -104     
+ Partials       1659          6      -1653     

Flag	Coverage Δ
integration	41.5623% <ø> (-6.6372%)	⬇️
unit	76.7648% <ø> (+0.3188%)	⬆️

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Components	Coverage Δ
dumpling	56.7974% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8077% <ø> (-12.3142%)	⬇️

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[hawkingrei]

/retest

[hawkingrei]

/ok-to-test

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-12 03:45:29.451995331 +0000 UTC m=+242773.513860267: ☑️ agreed by hawkingrei.

[hawkingrei]

/retest