feat: #3268207 Hand the entity being edited to the selection plugin

Issue: https://www.drupal.org/project/autocomplete_deluxe/issues/3268207

Builds on the patches from Robert, Guido and the retests: a selection handler gets the entity being edited, so it can do things like leave the entity out of its own suggestions.

• The widget puts the entity into the element's #selection_settings, so EntityAutocomplete::validateEntityAutocomplete() sees it too, and sends the entity type and id with the autocomplete request when the entity is saved.
• The controller loads the entity into the selection settings only when it loads and the user may view it, and an unknown entity type is caught rather than thrown, so a crafted request cannot 500 the route or smuggle an unviewable entity into a plugin.
• The hashed settings and the key value store keep leaving the entity out, so one field keeps one key in the store whatever entity is being edited.

Tested on a fresh Drupal 11 site built for this issue, with a small selection plugin that excludes the term named like the host entity: on the edit form of a node titled Alpha, typing Al offers Albion and Alps only; on the add form all three terms are offered. A request with an unknown entity type, another user's entity, or a missing id answers 200 and simply skips the entity.

AI-Generated: Yes

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