fix: resolve crashes in DiffStreamHandler — unsafe cast and negative line numbers (#11893)

* fix: resolve crashes in DiffStreamHandler — unsafe cast and negative line numbers

- Use safe cast (as?) for TextEditor in undoChanges() to prevent ClassCastException
- Add early return for empty documents and clamp line numbers to prevent IndexOutOfBoundsException in initUnfinishedRangeHighlights()
- Remove redundant .toList() call in rejectAll()

Fixes #4267

* refactor: inline lineNum variable in initUnfinishedRangeHighlights