[release-19.0] fix: handle info_schema routing (#15899)

[vitess-bot]

Description

This is a backport of #15899

[vitess-bot]

Review Checklist

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General

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[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 67.48%. Comparing base (5a5975e) to head (e447733).

Files	Patch %	Lines
go/vt/vtgate/planbuilder/vexplain.go	78.94%	4 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-19.0   #15906      +/-   ##
================================================
+ Coverage         67.45%   67.48%   +0.02%     
================================================
  Files              1560     1560              
  Lines            192812   192822      +10     
================================================
+ Hits             130055   130117      +62     
+ Misses            62757    62705      -52     

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