Clamp paste cursor position to document length (#8616)

The moveCursorByPasteLength extension could calculate a cursor position
beyond the document length after a paste, causing a RangeError. This
happens because the setTimeout callback reads the post-paste cursor
position and adds the paste length again, overshooting the document end.

Closes #8606

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/javascript/components/bootcamp/JikiscriptExercisePage/CodeMirror/extensions/move-cursor-by-paste-length.ts

@@ -11,7 +11,9 @@ export const moveCursorByPasteLength = EditorView.domEventHandlers({
       const pastedLength = pastedText.length
 
       view.dispatch({
-        selection: { anchor: from + pastedLength },
+        selection: {
+          anchor: Math.min(from + pastedLength, view.state.doc.length),
+        },
         scrollIntoView: true,
       })
     }, 0)