fix: prevent duplicate toast announcements

[wanxiankai]

Summary

• remove the redundant live-region attributes from ToastContainer
• keep each toast's configurable role as the single announcement source
• add a regression test that prevents toast live regions from being nested

Fixes #1291

Why

ToastContainer used aria-live="polite" while each toast defaults to
role="alert". Since alert is itself a live region, this nested live-region
structure caused NVDA to announce the same toast multiple times and announce it
again when it was removed.

Removing the container's live-region attributes avoids the duplicate
announcements without changing toast roles, custom role overrides, the
container's accessible label, or keyboard navigation.

Test plan

• pnpm build
• pnpm exec prettier --check src/components/ToastContainer.tsx src/tests.cy.tsx
• pnpm exec cypress run --component -b chrome --spec 'src/**/*.cy.tsx,packages/**/*.cy.tsx' (70 passing, 7 existing pending)
• Playground: verified one role="alert" toast with no aria-live ancestor