fix(core): add editor cleanup in BlockNoteEditor.test.ts to prevent unhandled DOMObserver errors (#2816)

Tests that mount editors were not calling unmount() afterward, leaving
ProseMirror's DOMObserver timers active. When prosemirror-view is updated
to newer versions (e.g. 1.41.8 via fresh dep resolution), a dangling
setTimeout in DOMObserver.flush() fires after jsdom teardown, causing
'document is not defined' ReferenceError.

Add afterEach cleanup following the same pattern as transformPasted.test.ts.