feat(data-grid): add cell/row accessibility prop callbacks

[hovoaep]

Problem

The hidden accessibility <table> tree rendered alongside the canvas (used by screen readers) offered no way to customize its markup per cell/row, and had an unrelated correctness bug for custom cells:

1. No a11y customization hooks. The <td>/<tr> elements in the accessibility tree only ever got the grid's built-in attributes (role, aria-selected, aria-readonly, aria-rowindex, etc.). There was no way for consumers to add supplemental aria-* attributes (e.g. aria-label, aria-describedby) to a specific cell or row.
2. Custom cells never used their renderer's accessibility string. getRowData special-cased GridCellKind.Custom to always return cell.copyData, without ever asking the resolved CustomRenderer for a proper accessible description — and CustomRenderer didn't even expose a getAccessibilityString field to ask for.

Repro

• Register a CustomRenderer with a getAccessibilityString implementation and render a Custom cell → the accessibility <td> text content is always copyData, the renderer's string is silently ignored.
• Try to attach an aria-label to an individual cell or row → no prop exists for it; DataGridProps only exposes grid-wide callbacks (getCellContent, getRowThemeOverride, etc.), none scoped to a11y markup.

Fix

1. Add getCellAccessibilityProps?: (cell: Item) => React.TdHTMLAttributes<HTMLTableCellElement> and getRowAccessibilityProps?: (row: number) => React.HTMLAttributes<HTMLTableRowElement> to DataGridProps, threaded through the full render chain: DataGridDnd → ScrollingDataGrid → DataGridSearch → DataEditor.
2. Spread order matters. Caller-supplied props are spread after the grid's built-ins (role, aria-selected, aria-readonly, aria-rowindex) so consumers can override defaults, while props required for grid interactivity (id, onClick, onFocusCapture, ref, tabIndex) stay locked in after the spread.
3. Custom cell accessibility string fix. Added optional getAccessibilityString?: (cell: T) => string to the CustomRenderer interface. getRowData now resolves the renderer first and uses its getAccessibilityString when defined, falling back to cell.copyData otherwise.

Test

New tests in test/data-grid.test.tsx:

• getCellAccessibilityProps adds custom attributes and overrides a built-in (aria-readonly) on the a11y <td>.
• getRowAccessibilityProps adds custom attributes and overrides a built-in (aria-rowindex) on the a11y <tr>.
• Custom cell a11y string uses the renderer's getAccessibilityString when provided.
• Custom cell a11y string falls back to copyData when the renderer has no getAccessibilityString.