[slack-22.0]: backport 18967: connpool: fix connection leak during idle connection reopen

[tanjinx]

…eopen (vitessio#18967) (vitessio#18970)

Description

backport vitessio#18970 (vitessio#18967)

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.72%. Comparing base (b1ab347) to head (c184fae).
⚠️ Report is 20 commits behind head on slack-22.0.

Additional details and impacted files

@@              Coverage Diff               @@
##           slack-22.0     #758      +/-   ##
==============================================
+ Coverage       67.53%   69.72%   +2.19%     
==============================================
  Files            1600     1604       +4     
  Lines          261782   213319   -48463     
==============================================
- Hits           176786   148747   -28039     
+ Misses          84996    64572   -20424     

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