[6.x] Prevent bogus content when dragging inside a node-selected Bard set

[duncanmcclean]

This pull request fixes an issue where dragging a Grid row inside a Bard set could dump a copy of the set's content into the editor as a table.

This was happening because clicking a non-interactive area inside a set makes ProseMirror node-select it (the blue border). That node selection is backed by a real DOM selection spanning the whole set, which ProseMirror hides visually — so when you drag from anywhere inside the set (like a Grid row's drag handle), the browser starts a native drag of that invisible selection instead of the row sort. And since ProseMirror ignores dragstart events originating inside node views, the drag data ends up being the set's raw HTML, meaning Bard parses the Grid's <table> markup into an actual table node on drop.

This PR fixes it by preventing native dragstart events that originate inside the set while the document selection covers the whole set. Drags of genuinely draggable elements (like Bard images, or nested sets via their drag handles) are unaffected, as is dragging the set itself using its own drag handle.

Fixes #14920