fix(toolkit-lib): diff leaves a deleting shell stack behind, breaking the next deploy

[akagr]

Fixes #1993

cdk diff --method=change-set materialises a REVIEW_IN_PROGRESS shell stack for a stack that does not exist yet, then deletes it without waiting for the delete to complete. A cdk deploy started shortly afterwards still sees the shell via DescribeStacks, so canSkipDeploy calls template() → GetTemplate, which fails with Stack [<name>] does not exist because a deleting shell has no template. That error is not caught (lookup only matches the Stack with id <name> does not exist wording), so the deploy exits 1.

Deployments.cleanupChangeSet already waits after the equivalent delete; this makes createChangeSetAndCleanup do the same, and applies the same fix to createValidationChangeSet.

The wait is gated on a new stackExistedBefore flag rather than the existing exists, so a diff only ever waits for a shell that its own change set created. exists reports a stack in REVIEW_IN_PROGRESS/DELETE_IN_PROGRESS as absent, so gating on it would make a read-only diff block on an unrelated stack that someone else already had in flight. CreateChangeSetOptions is module-internal (not re-exported from lib/index.ts), so the added field is not an API change.

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

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