feat: Add TableSortButton for atomic table components - #5269

Description

Add TableSortButton, a sort control for a TableHeaderCell in the table atomic components. Consumers no longer have to build their own sort button.

• Props: sortingState (sortable, ascending or descending), disabled, ariaLabel, onSortingChange with the next state, and the label as children.
• It looks like the existing Table's sort control: the label truncates with an ellipsis, the caret never shrinks, sorted and hovered columns use the active colour, and it has the Cloudscape focus ring.
• TableHeaderCell sets its own aria-sort from the button's state. An explicit ariaSort on the cell still wins.
• The existing Table and TableSortButton now share one internal sort icon, so IconProvider's table.sortingIndicator customises both. The Table's DOM is unchanged.
• New test utils: findLabel(), findSortingIcon(), getSortingState() and isDisabled().
• The table-root/sorting dev page now uses it, with an unsortable column and its state in URL parameters.

API proposal: TfIwIU4ePrLh

Related links, issue #, if available: n/a

How has this been tested?

• New unit tests: the label renders in a native button; each sort state shows its indicator and sets aria-sort on the header cell, including after the state changes; an explicit ariaSort wins; activating requests the next state; disabled blocks activation and shows the indicator only when sorted; ariaLabel; a custom indicator from IconProvider gets each state; and a11y validation.
• Existing Table, table-root, table-header-cell and icon-provider unit tests pass. The documenter and test-utils snapshots only add the new component.
• Manually, on the table-root/sorting dev page, in visual refresh: the sortable, sorted, hover and keyboard focus states.

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