feat(live-preview): expose preview status helper [SPA-5289]

[Maxim Cheremisin (maxcheremisin)]

Summary

Adds a public status signal for the Preview Session transport and wires it into the React live-preview hook.

Description

The shared live-preview client exposes subscribeStatus. The exported sendPreviewStatus helper publishes this message to the containing preview host:

{
  source: 'experiences/live-preview',
  type: 'status',
  status: 'live' | 'static',
}

useLivePreview
  ├─ subscribeStatus(sendPreviewStatus)
  │   ├─ missing sessionId or previewToken → static
  │   └─ WebSocket opens → live
  └─ subscribe()
      └─ Preview Session update → refresh experience data

The React adapter connects the existing client lifecycle to the host-status helper. The generic package owns the message format, while the adapter only wires it into useLivePreview.

Relevant code:

• packages/live-preview/src/live-preview-client.ts — status subscription and transport status
• packages/live-preview/src/preview-status.ts — host-message helper
• packages/adapter-react/src/use-live-preview.ts — React integration

Status updates are deduplicated, and reconnecting preserves the live-preview status.

This PR is stacked on #165 (feat/SPA-5161-live-preview-adapters).

Motivation and Context

The containing preview host needs to know whether the SDK is using static preview updates or an active Preview Session connection. A shared helper keeps this protocol consistent for direct package users and framework adapters.

Validation

• npx nx test live-preview — 36 tests
• npx vitest run src/use-live-preview.test.tsx — 7 tests

PR Checklist

• I have read the CONTRIBUTING.md file
• All commits follow conventional commits
• Documentation is updated (if necessary)
• PR doesn't contain any sensitive information
• There are no breaking changes

Note: This PR was created with the contentful-github-create-pull-request skill, powered by Agents Kit. To follow or use this workflow, see the Agents Kit CLI skill docs.

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