Normalize non-text selection ranges in gridCore getSelectionRange

[Copilot]

This addresses the review feedback on PR comment r3590182779: getSelectionRange declared numeric SelectionRange values but could return undefined for non-selectable active elements. The change keeps the contract sound by normalizing unsupported selection values.

• Problem addressed

  • getSelectionRange could return { selectionStart: undefined, selectionEnd: undefined } when the focused element did not support text selection APIs.
  • Downstream logic already treats negative indices as “no selection”; returning undefined violated the expected numeric shape.

• Targeted code change

  • Updated /packages/devextreme/js/__internal/grids/grid_core/m_utils.ts in getSelectionRange(...).
  • selectionStart / selectionEnd are now normalized to -1 when values are not numeric.

• Behavioral impact

  • The function now consistently returns numeric selection bounds for supported and unsupported elements.
  • Existing guards using >= 0 continue to work without shape mismatches.

return {
  selectionStart: isNumeric(focusedElement.selectionStart) ? focusedElement.selectionStart : -1,
  selectionEnd: isNumeric(focusedElement.selectionEnd) ? focusedElement.selectionEnd : -1,
};