feat(Headless): #3591962 Enable page variants in Canvas Headless

• adds support for page variants in Canvas Headless content api, editor preview and headless SDK

Testing instructions

• 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
    • for nextjs template use npm run dev -- --webpack, turbopack doesn't work with linked dependencies
• configure headless frontend in Canvas
• verify that page templates can be created/edited using headless editor frame
• verify that when previewing in Canvas editor pages or full content templates their assigned or site default page template is displayed around their content
• verify that content api returns in content full component trees including page templates
  • verify that page templates are displayed for published pages viewed in frontend app

Closes #3591962

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