[release-19.0] Fix regression where inserts into reference tables with a different name on sharded keyspaces were not routed correctly. (#15796)

[vitess-bot]

Description

This is a backport of #15796

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 67.47%. Comparing base (2c42825) to head (3b613eb).

Files	Patch %	Lines
.../vt/vtgate/planbuilder/operators/route_planning.go	77.77%	2 Missing ⚠️
go/vt/vtgate/planbuilder/operators/update.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-19.0   #15860      +/-   ##
================================================
- Coverage         67.47%   67.47%   -0.01%     
================================================
  Files              1560     1560              
  Lines            192780   192794      +14     
================================================
+ Hits             130076   130084       +8     
- Misses            62704    62710       +6     

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[harshit-gangal]

we should not panic here and return vterrors.VT13001

[rohit-nayak-ps]

As discussed, this function doesn't return an error and the standard pattern used is to panic here and catch panics at an outer level.

[harshit-gangal]

one comment. Rest looks good.