Issue #3575965 by scontzen: AJAX callback fails with Access denied for blocks...

Issue #3575965 by scontzen: AJAX callback fails with Access denied for blocks with path visibility conditions

Use plugin access check instead of full block entity access in the AJAX callback. The entity-level access check re-evaluates visibility conditions (e.g. request_path) against the AJAX route URL instead of the original page, causing the callback to always fail for blocks with path restrictions.

Closes #3575965