DataGrid - decompose _getNextCell method in keyboard navigation controller

[dmirgaev]

No description provided.

[Copilot]

Pull request overview

This PR refactors DataGrid keyboard navigation by decomposing the _getNextCell logic into smaller helper methods and tightening TypeScript typings for navigation-related parameters.

Changes:

• Introduced extended keyboard navigation direction and element type aliases in types.ts.
• Refactored _getNextCell into _adjustCellOnVerticalNav, _resolveInvalidCell, and _getNextCellSkippingHiddenRow helpers.
• Minor TypeScript type style cleanup in keyboard navigation integration tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/devextreme/js/__internal/grids/grid_core/keyboard_navigation/types.ts	Adds shared type aliases for extended navigation directions and element types.
packages/devextreme/js/__internal/grids/grid_core/keyboard_navigation/m_keyboard_navigation.ts	Decomposes _getNextCell and adds stricter typing around navigation params/return values.
packages/devextreme/js/__internal/grids/grid_core/keyboard_navigation/tests/keyboard_navigation.integration.test.ts	Adjusts generic type syntax style in it.each cases (no behavior change).

Comments suppressed due to low confidence (1)

packages/devextreme/js/__internal/grids/grid_core/keyboard_navigation/m_keyboard_navigation.ts:1705

• $cell is now typed as dxElementWrapper | null, but it’s used immediately via $cell.parent() without a null-guard. In the base controller _getFocusedCell() always returns a wrapper ($(...)), so the nullable type (and the subsequent $cell && check) is misleading. Either keep $cell as dxElementWrapper (preferred) or add an early return/guard before any $cell.* usage if null is actually possible.

      let $cell: dxElementWrapper | null = this._getFocusedCell();
      const isEditing = this._editingController.isEditing();

      if (!this.getMasterDetailCell($cell) || this._isRowEditMode()) {
        if (this._hasSkipRow($cell.parent())) {