Free-form answer input for ask-user questions.

Adds two behaviors over the shared base:

• When the cursor is on the first or last line of a choice question's Other free-text input, Up/Down are handed back to the enclosing choice list instead of moving the text cursor.
• Escape deselects the checked custom multi-select option that owns this field (via the uncheck_other binding), instead of cancelling the whole prompt. check_action keeps the binding unmatched otherwise, so Escape still bubbles to AskUserMenu.action_cancel.