Fix subquery merging regression introduced in #11379

[systay]

Description

This PR restores subquery merging behavior that was unintentionally broken by #11379, undoing the regression that reintroduced suboptimal query planning previously addressed in #10966.

Background

Given a query like:

SELECT music.id
FROM music
WHERE music.id IN (
  SELECT MAX(music.id)
  FROM music
  WHERE music.user_id IN (5, 6)
  GROUP BY music.user_id
)

We expect the planner to merge the subquery and send at most one query per shard based on the music.user_id IN (5, 6) condition. Currently, however, the planner fails to do this and ends up sending multiple queries per shard.

Curiously, wrapping the subquery in a derived table works as expected:

SELECT music.id
FROM music
WHERE music.id IN (
  SELECT * FROM (
    SELECT MAX(music.id)
    FROM music
    WHERE music.user_id IN (5, 6)
    GROUP BY music.user_id
  ) AS _inner
)

This PR restores the merging behavior so that both forms behave consistently and optimally.

Related Issue(s)

Fixes #18259

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.48%. Comparing base (421aca0) to head (9d27a15).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18260      +/-   ##
==========================================
+ Coverage   67.47%   67.48%   +0.01%     
==========================================
  Files        1602     1602              
  Lines      262191   262232      +41     
==========================================
+ Hits       176907   176970      +63     
+ Misses      85284    85262      -22     

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