Cannot place cursor between `.attachment-gallery` and `figure.attachment`

[shved270189]

The user can place the cursor between div.attachment-gallery and figure.attachment.

Steps to reproduce:

1. Upload 3 or more images.
2. Move one image below.
3. Try to place the cursor between the moved image and the images above.

Actual behavior:

The user cannot place the cursor here; instead, the cursor is placed below the moved image, not above it.

Expected behavior:

The cursor is placed at that position, similar to how it works between 2 figure.attachment:

[a-abdellatif98]

I'd like to work on this, together with #1131, which looks like the same underlying problem seen from a different angle: no valid cursor position exists adjacent to a block-level attachment.

Will write failing Playwright tests for both cases first and confirm whether one fix covers them.