Add test suite for `cc-grid` keyboard interactions

[pdesoyres-cc]

The cc-grid component implements comprehensive keyboard navigation following ARIA grid patterns, but currently lacks a dedicated test suite for these interactions. Adding tests would help ensure accessibility compliance and prevent regressions.

Keyboard interactions to test

Arrow key navigation:

• ArrowUp / ArrowDown - Navigate between rows
• ArrowLeft / ArrowRight - Navigate between columns
• Navigation should skip hidden (volatile) columns

Home/End navigation:

• Home - Move to first cell in current row
• End - Move to last cell in current row
• Ctrl+Home / Cmd+Home - Move to first cell in first row
• Ctrl+End / Cmd+End - Move to last cell in last row

Focus management:

• Focus should be restored correctly when grid items change
• Focus should move appropriately when a volatile column is hidden
• Sortable headers should be navigable (row index -1)
• Focus should be set correctly on focusable elements within cells (buttons, links, clipboard)