[release-23.0] planbuilder: fix subquery arg name collision across pullout opcodes (#19781)

[vitess-bot]

Description

This is a backport of #19781

[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 19864
git reset --hard origin/release-23.0
git cherry-pick -m 1 67a0d78ec3175c57823cc9a79c7052cfe37bc9c9

[Copilot]

Pull request overview

Backports a fix to prevent bind variable name collisions when the same subquery text appears in different pullout contexts (e.g., scalar value vs IN), ensuring each subquery occurrence gets its own reserved bind variable name and correct type.

Changes:

• Reserve a fresh bind variable name per subquery occurrence via ReserveSubQuery() (instead of caching via GetReservedArgumentFor).
• Track merged subqueries by arg name (map[string]*sqlparser.Subquery) to decouple merging logic from the reserved-argument cache.
• Update/add planbuilder golden test cases to assert distinct bind var names for duplicate/volatile subqueries and mixed scalar/IN scenarios.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/vt/vtgate/planbuilder/testdata/select_cases.json	Updates projection subquery pullout expectations; adds volatile uuid() subquery case asserting non-coalescing.
go/vt/vtgate/planbuilder/testdata/filter_cases.json	Adds WHERE-case asserting distinct arg names for same subquery used in scalar comparison and IN.
go/vt/vtgate/planbuilder/testdata/dml_cases.json	Adds UPDATE-case asserting distinct arg names for same subquery used in scalar SET and IN SET.
go/vt/vtgate/planbuilder/testdata/onecase.json	Normalizes file formatting (trailing newline).
go/vt/vtgate/planbuilder/plancontext/planning_context.go	Changes MergedSubqueries to a map keyed by arg name; removes subquery case from GetReservedArgumentFor.
go/vt/vtgate/planbuilder/operators/subquery_planning.go	Reworks merged-subquery rewrite/settling logic to use the arg-name-keyed map.
go/vt/vtgate/planbuilder/operators/subquery_builder.go	Ensures each subquery occurrence gets a unique reserved name; replaces old extraction/dedup flow with a single AST walk.
go/vt/vtgate/planbuilder/operators/subquery.go	Updates merged check to use MergedSubqueries map keyed by arg name.