[release-22.0] VStream API: Reset stopPos in catchup (#18119)

[vitess-bot]

Description

This is a backport of #18119

[vitess-bot]

Review Checklist

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General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

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Backward compatibility

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[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (release-22.0@db2c533). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...o/vt/vttablet/tabletserver/vstreamer/uvstreamer.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##             release-22.0   #18123   +/-   ##
===============================================
  Coverage                ?   67.55%           
===============================================
  Files                   ?     1600           
  Lines                   ?   261365           
  Branches                ?        0           
===============================================
  Hits                    ?   176568           
  Misses                  ?    84797           
  Partials                ?        0           

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