fix(docs): make the TreeView Custom onAction example selectable and non-disruptive

[mateoviilla1]

Summary

Fixes the Custom onAction TreeView example, which had two problems reported
in an accessibility pass (#2519). Both were in the example itself — no component
change.

The two issues

Additional checkboxes could not be selected. The example rendered checkboxes
but ran in selectionMode="single", so selecting a second item silently cleared
the first. With a checkbox affordance that reads as "nothing happened". It now
uses selectionMode="multiple", which matches what the checkboxes imply.

Activating an item opened an unannounced dialog. The handler called
window.alert. A modal dialog on a checkbox toggle is an unannounced context
change, which is disorienting for keyboard and screen reader users. The handler
now writes into a role="status" region, so the action is announced without
stealing focus.

const [lastAction, setLastAction] = useState(null);
…
<TreeView selectionMode="multiple" onAction={setLastAction} items={items} />
<p role="status">
  {lastAction ? `Last action: item ${lastAction}` : 'No action yet.'}
</p>

alert() is used in other docs examples (e.g. ActionMenu), and that is fine
there — the affordance is a menu item, where activating is expected to do
something. On a checkbox it is not.

Notes

• Needed import { useState } from 'react' at the top of the mdx; hooks are not
  automatically in scope in docs examples (same pattern as Accordion.mdx).
• Pre-existing on develop — the example came in with feat(TreeView): cascading selection (cascadeSelect / cascadeDeselect) and disabled nodes #2427. This is
  independent of the TreeView virtualization work in feat(TreeView): add opt-in virtualization via height prop #2475, which does not touch
  this example.

Testing

• Docs build (NODE_ENV=production webpack --bail) passes.
• Verified locally: checkboxes now toggle independently, and the status line
  updates without a dialog.

QA notes: on TreeView → Custom onAction, confirm several checkboxes can be
checked at once and that activating an item updates the status line instead of
opening a pop-up.

Ref #2519

[Copilot]

Pull request overview

Updates the TreeView documentation’s Custom onAction example to align behavior with its checkbox UI and to avoid disruptive modal dialogs during interaction, addressing the a11y issues reported in #2519 without changing the TreeView component itself.

Changes:

• Import useState in the MDX module so the live example can track action state.
• Update the “Custom onAction” example to use selectionMode="multiple" so multiple checkboxes can be selected.
• Replace window.alert with a role="status" region that announces the last activated item without stealing focus.

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