[FEATURE] Allow `preview` on `multiSelect` AskUserQuestion questions

Request

AskUserQuestion's per-option preview field is currently single-select only — a question with multiSelect: true cannot use it. Please allow it on both selection modes.

Why

Multi-select is exactly where per-option material is most valuable. With single-select the user picks one thing and can usually hold it in their head; with multi-select they are evaluating N options against each other and against the overlap between them, with no way to see what any individual option actually contains.

Concrete case

Presenting a set of adversarial review findings for accept/decline — a natural multiSelect — where each finding has a body worth showing: the mechanism it identifies, the file and line it points at, and the repair it proposes. With preview unavailable, all of that has to be compressed into the option description, which is effectively a one-liner; the alternative is dumping every finding into the shared question text, which makes the prompt unreadable long before you reach four options.

The same shape shows up for any "pick which of these to apply" prompt: selecting a subset of migrations, lint fixes, or config changes, where the user needs to see each candidate's content to choose the combination.

Notes

• The rendering surface already exists and is actively maintained (see #88206), so this looks like lifting a schema restriction rather than building new UI. The tool docs state the side-by-side preview layout activates "when any option has a preview" — that layout should work equally well against a checkbox list as against a radio list.
• Related but distinct from #77538, which asks for a shared per-question evidence/context payload rendered alongside the options. This issue is narrower: only about letting the existing per-option field cover both selection modes. Either can land without the other.