[release-22.0] bugfix: Plan group by only on top of derived tables correctly (#18505)

[vitess-bot]

Description

This is a backport of #18505

[vitess-bot]

Review Checklist

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Backward compatibility

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.54%. Comparing base (6b936ba) to head (2e69230).
⚠️ Report is 6 commits behind head on release-22.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-22.0   #18511      +/-   ##
================================================
- Coverage         67.54%   67.54%   -0.01%     
================================================
  Files              1600     1600              
  Lines            262004   262004              
================================================
- Hits             176974   176964      -10     
- Misses            85030    85040      +10     

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