feat: support focusable disabled collection items via allowFocusWhenDisabled

[Wonchang0314]

Closes #10276

📝 Summary

MenuItem, ListBoxItem, Tab, and TreeItem could not implement the ARIA APG Focusability of disabled controls pattern. A disabled collection item was stuck between two behaviors:

disabledBehavior	focusable / navigable	aria-disabled
'all' (default)	❌ removed from nav	✅
'selection'	✅	❌

Neither produces a focusable item that is also marked aria-disabled (unlike Button, which decouples aria-disabled from focusability via allowFocusWhenDisabled).

This PR adds an opt-in, per-item allowFocusWhenDisabled prop to these four components. The item stays fully non-interactive (no selection, no action, aria-disabled still emitted) but remains reachable via roving tabindex / arrow-key navigation.

🔧 Approach

A single isDisabled(key) currently drives three concerns at once: aria-disabled, selection/action blocking, and removal from keyboard navigation. To support focusable-disabled items without changing any of that existing behavior, this PR leaves isDisabled untouched and adds one independent axis that only controls focusability:

• react-stately: new optional MultipleSelectionManager.isFocusableWhenDisabled(key), backed by the new item prop.
• useSelectableItem: keeps the roving tabindex/focus wiring for these items; selection/action stay blocked and aria-disabled stays set. (Native <a href> navigation is already prevented by the existing link onClick, so no extra handling is needed.)
• List/Grid/Tabs keyboard delegates & useTab: stop skipping / un-tabbing focusable-disabled items.
• react-aria-components: expose allowFocusWhenDisabled on MenuItem, ListBoxItem, Tab, TreeItem.

The decoupling is inspired by base-ui's focusableWhenDisabled and this library's own Button allowFocusWhenDisabled. Unlike base-ui, which makes composite items focusable-when-disabled by default, this is strictly opt-in so existing apps relying on disabled items being skipped are unaffected. (Type-to-select also reaches these items, consistent with them being focusable.)

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue (aria-disabled is not passed to MenuItem/ListBoxItem/Tab/TreeItem #10276).
• Added/updated unit tests for this change.
• Filled out test instructions.
• Updated documentation (docs follow-up if desired).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Automated (all green):

yarn jest packages/react-aria-components/test/{Menu,ListBox,Tabs,Tree}.test.*

Each component has new tests (covering both the item-level isDisabled prop and collection-level disabledKeys paths) asserting that an item with allowFocusWhenDisabled: (1) keeps aria-disabled="true", (2) is reachable via arrow-key navigation (not skipped), (3) cannot be selected or actioned.

Manual:

<Menu aria-label="Actions" disabledKeys={['rename']}>
  <MenuItem id="open">Open</MenuItem>
  <MenuItem id="rename" allowFocusWhenDisabled>Rename</MenuItem>
  <MenuItem id="delete">Delete</MenuItem>
</Menu>

Arrow-key down should land focus on "Rename"; a screen reader should announce it as disabled; pressing Enter should do nothing.

🧢 Your Project:

Community contribution