Fix AVG() sharded planning

[GuptaManan100]

Description

This PR fixes the average planning bug found in #15625. The issue was with the simple projector, wherein we were creating a new incorrect field instead of reusing an already existing one.

Related Issue(s)

• Fixes Bug Report: AVG query in sharded setup with a group by without selecting group by columns returns incorrect results #15625

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 68.09%. Comparing base (cef95ab) to head (ca78415).
Report is 7 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/projection.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15626      +/-   ##
==========================================
+ Coverage   68.06%   68.09%   +0.02%     
==========================================
  Files        1561     1561              
  Lines      195288   195313      +25     
==========================================
+ Hits       132927   132993      +66     
+ Misses      62361    62320      -41     

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