[toast] Add upsert support to add method

[atomiks]

Closes #4228

https://deploy-preview-4440--base-ui.netlify.app/react/components/toast#deduplicated-toast

This adds built-in upsert behavior to toastManager.add() when callers pass an id, so repeated adds update the existing toast instead of creating a duplicate. Matched toasts keep their identity, refresh their auto-dismiss timer, and a toast that is already ending is replaced with a fresh one instead of being revived mid-dismissal.

It also adds updateKey to toast objects so custom renderers can replay styles when a toast is updated or upserted. The docs now include a deduplicated toast demo that uses updateKey to pulse the existing toast without remounting it.

Changes

• Treat add({ id }) as an upsert in the toast store and increment updateKey on updates and upserts.
• Add regression coverage for manager and hook flows, including timer refresh, re-adding while a toast is closing, and updateKey changes on add() and update().
• Document the id upsert behavior and add a deduplicated toast demo that replays a pulse via updateKey.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4440
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4440
  

commit: 1440cbc

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+294B(+0.06%)	🔺+107B(+0.07%)

Details of bundle changes

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