Guard caret checks until editor view mounts

[ComputelessComputer]

Consolidating #4499 and #4680. The enhanced summary flow can remount the editor and run caret-position effects before TipTap has attached its ProseMirror view, so editor.view.coordsAtPos throws the editor-view-not-available error. This likely means the mount effect is reading editor.view during the streaming-to-editor handoff without guarding that the view exists yet.