fix(react): keep portal root last in body

[raashish1601]

Summary

• keep the shared React portal root as the last child of document.body`n- repair external DOM reordering before the next portal render
• add a focused regression for the reordered portal-root case

Closes #3836.

Testing

• npx prettier --write packages/@headlessui-react/src/components/portal/portal.tsx packages/@headlessui-react/src/components/portal/portal.test.tsx
• npx jest --runInBand packages/@headlessui-react/src/components/portal/portal.test.tsx
• git diff --check

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