Gallery: Let a click on an overlaid caption reach the image - #85381

A gallery caption is laid over the bottom of its image and may grow to
cover it entirely. With "Enlarge on click" the lightbox opens from a
click on the `img`, so a caption that covered the image took every one
of those clicks and left no way to open it. The only way in was the
Enlarge button, which is invisible until the image is hovered, and so
absent on touch.

Let the whole caption click through to the image, as the blur scrim over
it already does. The tile becomes one surface carrying the zoom cursor
throughout, and the caption is read in the lightbox, where it is legible
and selectable, rather than four lines at a time in a thumbnail. This
takes in any link the caption holds: `pointer-events` is inherited, so a
link stops being clickable here too, which keeps one cursor and one
behaviour across the whole image.

The rounded and bordered styles put the caption under the image rather
than over it, where it covers nothing and there is nothing to click
through to, so they are excluded and keep their captions scrollable and
selectable. `wp-lightbox-container` is added when the block is rendered,
so captions in the editor, and in galleries without the lightbox, are
untouched as well.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>