Fix caret jumping to the title after inserting a Plone block

[sneridagh]

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Inserting a Plone block from the slash menu (Image, Teaser, Listing) left the caret at the start of the title instead of on the new block, so typing went into the title.

Two causes in plone-block-adapter.tsx:

• Lazy Edit components suspended the whole block. Block edit/view components are React.lazy. The first render suspended up to a boundary above the element, so the block element and Slate's spacer text were not in the DOM when the slash menu refocused the editor. ReactEditor.focus threw Cannot resolve a DOM node from Slate node before calling el.focus(), and the browser caret stayed in the title.
• The whole void element was contentEditable={false}, spacer included, so the browser could not place the caret on the block. Once the paragraph holding the slash command was removed, the caret fell back to the start of the editor. (This one shows up when the Edit component is already loaded, e.g. in Volto-based integrations of the editor.)

The fix gives the block's own UI its own Suspense boundary and moves contentEditable={false} onto a wrapper around it, so the element and its spacer render immediately and stay editable.

Tests

New acceptance test in packages/plate/acceptance/tests/slash-menu.test.ts, run for Image, Teaser and Listing: after inserting the block, the editor has focus and the DOM caret and Slate selection are on the new block. It fails on main and passes with this change. The full packages/plate acceptance suite (86) and unit tests (74) pass, and check:ts is clean.