test(rust): provider-react smoke against the Rust server

Mounts the real HocuspocusProviderWebsocketComponent + HocuspocusRoom in
jsdom, asserts the room syncs, that an independent plain provider observes
a change made through the React-managed document, and that unmounting
closes the server-side connection. provider-react previously had no test
coverage against either server implementation.

Adds jsdom/react/react-dom/tsx as tests devDependencies (react pinned to
the instance provider-react resolves, to avoid a duplicate-React tree).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SuJw4JrwBzqKtW3ZBuRdkP

Author: claude