[release-20.0] Fix vtexplain not handling UNION queries with weight_string results correctly. (#16129)

[vitess-bot]

Description

This is a backport of #16129

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.55%. Comparing base (d50d290) to head (6425fd5).
Report is 1 commits behind head on release-20.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16158      +/-   ##
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+ Coverage         68.47%   68.55%   +0.07%     
================================================
  Files              1541     1541              
  Lines            197623   197628       +5     
================================================
+ Hits             135331   135483     +152     
+ Misses            62292    62145     -147     

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