feat(Headless): #3591861 Use the new Canvas content API endpoint in the headless SDK

#3591861

• Switches the headless SDK's draft-aware fetchPage() client from /ce-api/{path} to /canvas/content-api.
• Adopts the Canvas endpoint's content, document-head, route, and redirect response contract.
• Removes the now obsolete Lupus endpoint dependencies and the /ce-api auto-save controller override.

SDK response handling

fetchPage(requestUri) now returns:

• Page, containing Canvas-rendered content when available, document-head data, and normalized route information. route.managedByCanvas indicates whether Canvas manages the route's complete component tree.
  • route.managedByCanvas is used to distinguish an empty Canvas-managed page from a route Canvas does not render, because both return content: null.
• PageRedirect, containing the redirect URL, status code, and external-URL flag.
• null when Drupal returns a non-success response.

isPageRedirect() distinguishes redirect results before page fields are read. Live draft requests continue to carry the preview bearer token. Empty draft content receives an editor-only structural root so the frontend can render an editable content-region placeholder.

Component-tree bindings now consume one structured root or null instead of the former structured-or-markup response.

Lupus removal

The SDK no longer calls /ce-api, so this MR removes:

• The drupal/lupus_decoupled development dependency.
• The lupus_ce_renderer and lupus_decoupled_ce_api module dependencies.
• The Lupus-specific auto-save preview controller, controller subscriber, services, and kernel test.

Testing

• build ui and packages and enable canvas_headless
  • npm run build
  • npm run packages:build
  • ddev drush cr
  • npm link -ws
  • ddev drush en canvas_headless
• create a headless app: npx @drupal-canvas/create@latest --experimental-headless --template nextjs
• start headless app
  • npm link @drupal-canvas/headless @drupal-canvas/headless-next @drupal-canvas/headless-react (if using a template other than the nextjs, link @drupal-canvas/headless and framework adapter matching the template)
  • npm run dev
• configure frontend in canvas
• verify that headless preview in canvas works and uses auto-save data
• verify that the headless app can display published canvas pages outside preview context

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AI-Generated: Yes (Used AI coding agent to work on implementation).