fix: Render the Portal probe span during SSR

[TrevorBurnham]

Description

Fixes AWSUI-62219.

Portal renders a hidden probe span on its first render, so the layout effect can read ref.current.ownerDocument and find the right document inside an iframe. That span was added in #214 and gated on typeof document !== 'undefined':

if (!activeContainer && typeof document !== 'undefined') {
  return <span ref={ref} style={{ display: 'none' }} />;
}

activeContainer is populated in a layout effect, so it's null on the first render. On the client that returns the span, but during SSR the guard fails and the component returns null. That causes React to emit th eerror

Hydration failed because the server rendered HTML didn't match the client

and discard and re-renders the subtree.

This affects every SSR consumer that renders a portal, including ones with no application-level workaround. Two paths in @cloudscape-design/components:

• A Modal with visible={false}, which many apps mount unconditionally.
• A Table with resizableColumns, via StickyHeader → Resizer → DragHandleWrapper → PortalOverlay → Portal.

Before #214, the only return was activeContainer && createPortal(children, activeContainer), which yielded null on both passes.

The fix here drops the typeof document condition so the span renders during SSR too, and both passes agree. The cost is one display: none span in the server HTML, replaced by the portal on the first commit. The iframe behavior #214 added is unchanged: the effect still reads the ref.

How has this been reproduced and tested?

With any React 18+ SSR setup:

// server
import { renderToString } from 'react-dom/server';
import Modal from '@cloudscape-design/components/modal';

renderToString(<Modal visible={false} header="Title">Body</Modal>);

// client — hydrating that markup
hydrateRoot(container, <Modal visible={false} header="Title">Body</Modal>);

Before, React logs the mismatch and shows an added hidden span under Portal in the diff. After, there's no mismatch.

This PR adds unit test coverage to prevent regressions: src/internal/portal/__tests__/portal.ssr.test.tsx.

[TrevorBurnham]

cloudscape-design/components#4908 should be merged first to prevent tests from breaking downstream.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.09%. Comparing base (d7cc26a) to head (98ff8b6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage   98.09%   98.09%           
=======================================
  Files          56       56           
  Lines        1787     1787           
  Branches      517      495   -22     
=======================================
  Hits         1753     1753           
  Misses         34       34           

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[TrevorBurnham]

I believe the dry run failure ("Components unit tests") is because cloudscape-design/components#4908 hasn't been merged yet.