Guard GridItem.getCellSize against stale row index during item disposal

[Copilot]

When a row is disposed while scrollbar state is changing, GridEditor may re-layout against that row and call GridItem.getCellSize. In that window, the item can already be removed from Grid.items, causing parent.getItem(index) to throw ERROR_INVALID_RANGE.

• Height computation hardening (GridItem#getCellSize)

  • Stops re-fetching the current item from Grid by row index.
  • Uses getHeight() from the receiver directly.
  • Adds a bounds guard for indexOfCurrentItem before any span traversal over subsequent rows; returns current cell width + own height when index is stale.

• Regression coverage

  • Adds testDisposeItem_WithEditorAndScrollbarRemoval_DoesNotThrow in GridFixedColumn_Test.
  • Reproduces the failure mode: edited row + vertical scrollbar visibility/values + row dispose.
  • Asserts disposal path no longer throws IllegalArgumentException.

int indexOfCurrentItem = parent.getIndexOfItem(this);
int itemCount = parent.getItemCount();
int height = getHeight();
if (indexOfCurrentItem < 0 || indexOfCurrentItem >= itemCount) {
    return new Point(width, height);
}

• Fixes Index out of bounds in GridItem.getCellSize #700