[release-19.0] vtexplain: Fix setting up the column information (#15275)

[vitess-bot]

Description

This is a backport of #15275

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

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

All modified and coverable lines are covered by tests ✅

Comparison is base (b352323) 67.29% compared to head (6f044e2) 67.30%.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-19.0   #15282   +/-   ##
=============================================
  Coverage         67.29%   67.30%           
=============================================
  Files              1558     1558           
  Lines            192072   192068    -4     
=============================================
+ Hits             129261   129270    +9     
+ Misses            62811    62798   -13     

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