Accessibility: Sudo mode focus needs to be updated

[GuySartorelli]

In sections protected by sudo mode, clicking "verify to continue" should move the focus to the password field input element.

This means keyboard users can expand the section and then immediately type their password.
It also means for screen reader users they don't suddenly lose their place.

Acceptance criteria

• For sudo mode, activating the "verify to continue" button should move focus to the newly displayed input element

Do not merge until CMS 6.1 beta has been released

Kitchen sink CI

• https://github.com/creative-commoners/recipe-kitchen-sink/actions/runs/16924623442

PRs

• NEW Add theFieldShouldHaveFocus method silverstripe-behat-extension#310
• ENH Focus the sudo mode field after clicking button #1976