bugfix: support subqueries inside subqueries when merging

[systay]

Description

Subqueries inside subqueries were not handled well if they could all be merged into a single query.

Related Issue(s)

Fixes #17775

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

All modified and coverable lines are covered by tests ✅

Project coverage is 67.94%. Comparing base (22cf8b6) to head (d54f622).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17764      +/-   ##
==========================================
- Coverage   67.95%   67.94%   -0.02%     
==========================================
  Files        1585     1585              
  Lines      255162   255169       +7     
==========================================
- Hits       173408   173382      -26     
- Misses      81754    81787      +33     

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