fix: rows affected count for multi table update for non-literal column value

[harshit-gangal]

Description

This PR fixes the calculation for rows affected for a update query with multi table when a column is updated with non-literal value.

Related Issue(s)

• Fixes multi-table join in sharded keyspace returns unexpected number of rows affected #16171

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.56%. Comparing base (8a01d03) to head (5c5deee).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16181      +/-   ##
==========================================
+ Coverage   68.54%   68.56%   +0.02%     
==========================================
  Files        1544     1544              
  Lines      197737   197737              
==========================================
+ Hits       135532   135583      +51     
+ Misses      62205    62154      -51     

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[systay]

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