The CodeHighlightPlugin's selection restoration could create an invalid
Lexical selection targeting a LineBreakNode with type 'element'. Since
LineBreakNode is not an ElementNode, Lexical threw during reconciliation
and the LexicalErrorBoundary replaced the editor with an error state.
The fix ensures findPoint never targets a LineBreakNode directly.
Instead it falls back to an element-level selection on the parent
CodeNode using the child index, which is always a valid target.
Extracted the selection helpers (getCodeNodeAbsoluteOffset,
findCodeNodeSelectionPoint, restoreCodeNodeSelection) into
markdownEditor.ts for testability and added regression tests.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
