[toast] Add focus option to move focus to toast on open

[Jihye2ee]

Summary

Adds a focus option to toastManager.add() that moves focus to the toast when it opens. This enables keyboard and screen reader users to reach interactive actions (e.g. Undo) without relying on the F6 shortcut.

Closes #4253

Changes

• useToastManager.ts — Add focus?: boolean to ToastObject
• ToastRoot.tsx — On mount, if focus is true: save prevFocusElement, focus the toast, pause timers, and set focused state (same pattern as F6 in ToastViewport)
• ToastRoot.test.tsx — Add tests for focus behavior
• Undo demo — Apply focus: true to demonstrate the pattern

How it works

1. toastManager.add({ focus: true, ... })
2. Toast mounts → useEffect saves prevFocusElement, focuses the toast
3. Viewport's handleFocus pauses timers (existing infra)
4. User interacts (Undo/Close) or tabs away → timers resume (existing infra)
5. On close → handleFocusManagement restores focus (existing infra)

When focus is not set, behavior is identical to before — no breaking changes.

Screenshot

Tab → Undo → Click	Tab → Undo → Tab out → auto-dismiss

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4533
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4533
  

commit: 3d20398

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+171B(+0.04%)	🔺+45B(+0.03%)

Details of bundle changes

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