[release-21.0] Fix to prevent stopping buffering prematurely (#17013)

[vitess-bot]

Description

This is a backport of #17013

[vitess-bot]

Review Checklist

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General

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[codecov]

Codecov Report

Attention: Patch coverage is 74.71264% with 22 lines in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (a718a56) to head (14ab308).
Report is 2 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/vtgate/buffer/shard_buffer.go	40.00%	9 Missing ⚠️
go/vt/discovery/keyspace_events.go	85.71%	7 Missing ⚠️
go/vt/discovery/fake_healthcheck.go	66.66%	4 Missing ⚠️
go/vt/vtgate/buffer/buffer.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17205      +/-   ##
================================================
+ Coverage         67.50%   67.53%   +0.02%     
================================================
  Files              1570     1570              
  Lines            251294   251373      +79     
================================================
+ Hits             169633   169759     +126     
+ Misses            81661    81614      -47     

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