[release-22.0] fix: keep LIMIT/OFFSET even when merging UNION queries (#18361)

[vitess-bot]

Description

This is a backport of #18361

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.58%. Comparing base (e361adb) to head (33b517f).
Report is 1 commits behind head on release-22.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-22.0   #18363      +/-   ##
================================================
+ Coverage         67.55%   67.58%   +0.02%     
================================================
  Files              1600     1600              
  Lines            261860   261861       +1     
================================================
+ Hits             176907   176972      +65     
+ Misses            84953    84889      -64     

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