feat: Add CKEditor 5 SharePoint file search plugin

Fabian de Rijkrequested to merge
feature/ckeditor5 into 6.0.x

Replaces the CKEditor 4 plugin with a CKEditor 5 implementation that adds a SharePoint toolbar button. Clicking the button opens a contextual balloon with a search input that queries the existing SharePoint autocomplete endpoint. Selecting a result inserts a link into the editor.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

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