simplify merging logic

[systay]

Description

This PR introduces a refined solution to the issue where vtgate stopped accepting cross-keyspace queries. In the original fix, we incorrectly rejected subqueries that weren't merged and failed to properly handle non-correlated subqueries from different keyspaces. This updated approach corrects the handling of these subqueries by planning them differently, ensuring that cross-keyspace functionality is restored in a more accurate and efficient manner.

Related Issues

This PR supersedes the previous fix and addresses the same underlying problem introduced in #12197 and initially addressed in #16515.

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

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.75%. Comparing base (eb29999) to head (c045042).
Report is 69 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16525      +/-   ##
==========================================
+ Coverage   68.69%   68.75%   +0.05%     
==========================================
  Files        1547     1556       +9     
  Lines      198297   199705    +1408     
==========================================
+ Hits       136228   137312    +1084     
- Misses      62069    62393     +324     

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[harshit-gangal]

looks good, one e2e or planning test for the fix would work.