[release-23.0] planner: substitute merged DML IN/NOT IN subqueries via ListArg placeholder (#19973)

[vitess-bot]

Description

This is a backport of #19973

[vitess-bot]

Hello @arthurschreiber, there are conflicts in this backport.

Please address them in order to merge this Pull Request. You can execute the snippet below to reset your branch and resolve the conflict manually.

Make sure you replace origin by the name of the vitessio/vitess remote

git fetch --all
gh pr checkout 20029
git reset --hard origin/release-23.0
git cherry-pick -m 1 5ea4edd93c7fb4a489382d663304ac3410b8ce3b

[Copilot]

Pull request overview

Backports a vtgate planner correctness fix to ensure merged DML IN/NOT IN subqueries in UPDATE ... SET expressions are properly de-placeholdered when those subqueries are merge-compatible with the outer route, avoiding broken emitted SQL that would reference missing bind vars.

Changes:

• Extend rewriteMergedSubqueryExpr to also substitute sqlparser.ListArg placeholders (used by DML IN/NOT IN) in addition to *sqlparser.Argument.
• Add regression coverage for UPDATE ... SET col = (1 IN (subquery)) and UPDATE ... SET col = (1 NOT IN (subquery)) where the subquery merges into the outer route.

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 sqlparser.ListArg handling so merged DML IN/NOT IN subqueries are rewritten back to their original inline subquery AST.
go/vt/vtgate/planbuilder/testdata/dml_cases.json	Adds two planner regression cases covering merge-compatible IN and NOT IN subqueries inside UPDATE ... SET.

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