Set parsed comments in operator for subqueries (#18369)

[mhamza15]

Description

This is a backport of vitessio#18369

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[Copilot]

Pull Request Overview

This pull request ensures that parsed comments are properly preserved when merging subqueries with outer queries in the Vitess query planner. The change specifically sets the comments from the outer Route operator to the merged operator when subqueries are merged.

• Preserves SQL comments during subquery merging operations
• Adds test coverage for comment handling in various subquery scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/vt/vtgate/planbuilder/operators/subquery_planning.go	Adds logic to preserve comments from outer Route when merging subqueries
go/vt/vtgate/planbuilder/testdata/select_cases.json	Adds test cases to verify comment preservation in subquery scenarios

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