feat: add selectable variant to List component

[ernst-dev]

Description

Adds an opt-in selectable variant to the List component (SIM AWSUI-62052), enabling users to choose from a set of predefined options without reaching for Table or Cards for simple selectable lists.

New (fully backward-compatible) props:

• selectionType?: 'single' | 'multi' — enables selection. When unset, List behaves exactly as before.
• selectedItems?: ReadonlyArray<T> — controlled selection (items matched by the id returned from renderItem).
• onSelectionChange?: NonCancelableEventHandler<ListProps.SelectionChangeDetail<T>> — fires with the updated selectedItems.
• isItemDisabled?: (item: T) => boolean — prevents selection of individual items.

Behavior / accessibility:

• When selectionType is set (and the list is not sortable), the container renders as role="listbox" with aria-multiselectable for multi-select, and each item renders as role="option" with aria-selected / aria-disabled.
• Keyboard support: roving tabindex, Up/Down/Home/End to move focus, Space/Enter to toggle selection.
• multi renders a checkbox control (reusing the internal CheckboxIcon); single renders a radio control. Controls are aria-hidden since state is conveyed via aria-selected.
• Selection is not combined with sortable; when both are provided, sorting takes precedence.

Test-utils: ListItemWrapper.findSelectionControl() and ListWrapper.findSelectedItems() added (dom + selectors).

Related links, issue #, if available: AWSUI-62052

How has this been tested?

• New unit test suite src/list/__tests__/list-selectable.test.tsx (11 tests): listbox/option roles, aria-multiselectable, controlled selection, single vs multi toggle semantics, disabled items, Space/Enter toggle, roving tabindex + arrow-key focus, and sortable-takes-precedence. All 31 List unit tests pass.
• New dev page pages/list/selectable.page.tsx demonstrating controlled single and multi selection with disabled items.
• quick-build, eslint, and stylelint are green. Documenter API + test-utils snapshots regenerated.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

❌ Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.61%. Comparing base (f97729a) to head (d2e0cdf).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/list/internal.tsx	97.18%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4817      +/-   ##
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- Coverage   97.61%   97.61%   -0.01%     
==========================================
  Files         952      952              
  Lines       30816    30891      +75     
  Branches    11318    11351      +33     
==========================================
+ Hits        30081    30154      +73     
- Misses        688      690       +2     
  Partials       47       47              

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