Reference Table DML Join Fix

[harshit-gangal]

Description

This PR fixes the issue with join query with reference table and sharded query.
An update/delete on reference table does not get routed to the source table.
This change checks the update/delete target and does not choose the alternate route of the query.

Backport Reason: Issue started after supporting join queries in DML from v20. This is a correctness issue as reference table is updated instead of the source table.

Related Issue(s)

• Fixes Bug Report: Routing DML of reference tables to the source fails when it's part of a join #17395

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 59.37500% with 13 lines in your changes missing coverage. Please review.

Project coverage is 67.63%. Comparing base (fa79c3f) to head (0fc2fe9).

Files with missing lines	Patch %	Lines
go/vt/sqlparser/ast_funcs.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17414      +/-   ##
==========================================
- Coverage   67.64%   67.63%   -0.02%     
==========================================
  Files        1583     1583              
  Lines      254043   254058      +15     
==========================================
- Hits       171854   171837      -17     
- Misses      82189    82221      +32     

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