#3604017 Email confirm metadata missing

A webform_email_confirm element's authored #description, #help, #help_title and #required are relocated onto its mail_1 sub-input (nested under flexbox when #flexbox is set) by processWebformEmailConfirm(), so the built parent element the schema walks no longer carries them and the metadata resolves empty. #required_error stays on the parent and still resolves, as the control.

Asserts the intended restored behaviour (red): EmailConfirmTest covers the real flexbox fixture element end-to-end, and WebformElementPropertyTest covers the flat and flexbox-nested mail_1 fallback branches in the producer.

Closes #3604017