MSMessageTemplateLayout
(the iMessage mini-app surface) — the iMessage provider renders it natively;
other platforms ignore it and fall back to the bare URL. At least one of
caption, subcaption, trailingCaption, trailingSubcaption, or image
must be set so the bubble is not empty — summary is the fallback text shown
on surfaces that cannot render the card and is not a visible slot on its own.
image and imageTitle must be set together; imageSubtitle requires
image.