Add ReflowUndo tests and integrate InsertRow undo support

- Added `test_reflow_undo.cc` to validate undo/redo workflows for reflow operations.
- Introduced `UndoType::InsertRow` in `UndoSystem` for tracking row insertion changes in undo history.
- Updated `UndoNode.h` and `UndoSystem.cc` to support row insertion as a standalone undo step.
- Enhanced reflow paragraph functionality to properly record undo/redo actions for both row deletion and insertion.
- Enabled legacy/extended undo tests in `test_undo.cc` for comprehensive validation.
- Updated `CMakeLists.txt` to include new test file in the build target.

CMakeLists.txt

@@ -337,6 +337,7 @@ if (BUILD_TESTS)
             tests/test_benchmarks.cc
             tests/test_migration_coverage.cc
             tests/test_smart_newline.cc
+            tests/test_reflow_undo.cc
 
             # minimal engine sources required by Buffer
             PieceTable.cc