fix: keep Modal portals accessible

[rswerve]

Summary

• defer portal creation until Modal mounts so server rendering never reads document
• keep the body child containing an open portal modal exposed to assistive technology
• cover both regressions and document the default #modal-root behavior

Related Issues or PRs

Closes #3575

How To Test

• npm test
• npm run lint
• npm run format:check
• npm run test:coverage
• npm run build
• npm run test:serverside
• npm run build-storybook

Screenshots (optional)

Not applicable.

Author & Maintainer checklist

• Is this is a breaking change?
  • Yes, and I accounted for the breaking changes according to the linked documentation
  • No
• Once merged, add the PR and Issue author(s) as a contributor(s) via the all-contributors bot

[github-actions]

	Warnings
⚠️	It looks like there are new component (JSX/TSX) files, but the entrypoint (index.ts) has not changed. - Did you forget to export new components from the library entrypoint?

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