bugfix: Handle CTEs with columns named in the CTE def

[systay]

Description

This PR addresses an issue where the Vitess planner was incorrectly handling recursive Common Table Expressions (CTEs) when column names were defined in the CTE declaration but not in the SELECT clause. Specifically, vtgate was stripping out the column names before passing the query to MySQL, resulting in query failures.

The fix ensures that column names defined in the CTE declaration are preserved, allowing recursive CTE queries to execute correctly in both Vitess and MySQL.

Related Issue(s)

Fixes #17177

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

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.33%. Comparing base (d9ab9f7) to head (dc0fce9).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17179      +/-   ##
==========================================
- Coverage   67.33%   67.33%   -0.01%     
==========================================
  Files        1569     1569              
  Lines      252476   252476              
==========================================
- Hits       170008   170003       -5     
- Misses      82468    82473       +5     

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