gen4 planner bugfix: issue when merging subqueries

[systay]

Description

In PR #12197, a refactoring error resulted in a bug causing the generation of invalid query plans under specific conditions.

While merging a subquery with the outer query, predicates from the subquery were not copied properly. This led to a partial merge where certain expressions were treated as unmergeable, creating an inconsistency. Consequently, invalid query plans were produced, resulting in the following error:

query arguments missing for __sq1

This PR resolves the issue and includes a test to prevent similar problems from occurring in the future.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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[arthurschreiber]

I tried backporting this into v16 locally, and I don't think the refactoring that broke this actually exists in v16. 🤦 Gonna remove the v16 backport label.