feat(Headless): #3591861 Add a Canvas-owned routed content endpoint for headless apps

#3591861

Adds a Canvas-owned endpoint for retrieving routed content:

GET /canvas/content-api?requestUri={requestUri}

The endpoint routes the supplied request URI through Drupal. It returns Canvas content only when Canvas manages the complete component tree. The response also contains an Unhead-compatible document head and normalized route information.

Matched routes that Canvas does not render still return a successful response with content set to null. Route-not-found and access-denied responses use RFC 9457 Problem Details.

Headless SDK support for the new endpoint is added in a separate MR: !1437

Comparison with /ce-api

Behavior	Lupus /ce-api/{path}	Canvas /canvas/content-api?requestUri={requestUri}
Ownership	Provided by lupus_decoupled_ce_api and lupus_ce_renderer	Provided by Canvas Headless using Custom Elements
Request mapping	Removes the /ce-api prefix and routes the remaining path through Drupal	Requires requestUri to be a path inside the Drupal site with no #fragment, then routes it through Drupal
Supported routes	Supports Drupal routes that Custom Elements can render	Routes accessible Drupal paths, but returns content only for canonical content entities with a complete Canvas tree or an enabled full content template
Unsupported routes	Returns a Custom Elements error page, normally with status 406; admin routes may redirect to Drupal	Returns content: null with available head and route data when Canvas cannot render the matched route; route and access failures use RFC 9457 Problem Details
Content formats	Supports Custom Elements as JSON or markup	Always returns the Custom Elements JSON shape, even when the site's global Custom Elements setting uses another format
Content roots	Canonical entity routes return one entity element; other routes may return a list when a top-level renderless-container is removed	Returns one element or null; multiple roots stay inside a transparent renderless-container wrapper
Content response	Returns title, breadcrumbs, metatags, content_format, content, page_layout, local_tasks, and messages	Returns content, head, and route
Document metadata	Returns the title and Metatag module data in Lupus-specific response fields	Returns title, meta, non-canonical, non-stylesheet links, and JSON-LD entries in a head object compatible with the Unhead library
Route information	Does not include route or entity details in a standard shape	Returns the route name, requested URI, route parameters, and rendered entity details when available
Redirects	Returns redirect and optional Drupal messages as JSON	Returns the redirect URL, original status code, and external URL flag as JSON
Error format	Renders Drupal error routes through the Custom Elements renderer	Returns application/problem+json responses
Partial responses	Supports _select=content	Returns the complete endpoint response, support for include param will be added later
Response identification	Adds X-Drupal-CE: page or X-Drupal-CE: redirect	Uses standard JSON and Problem Details response types
Draft previews	By default doesn't support Canvas auto-save data	Selects entity and content-template auto-save data for preview requests and checks entity access again after loading it
Extension points	Supports response overrides and hook_lupus_ce_renderer_response_alter()	Provides a focused API response owned by Canvas

Content response

{
  "content": {
    "element": "js-example-component"
  },
  "head": {
    "title": "Example page",
    "meta": [
      {
        "name": "description",
        "content": "Example description"
      },
      {
        "property": "og:title",
        "content": "Example page"
      }
    ],
    "script": [
      {
        "type": "application/ld+json",
        "textContent": {
          "@context": "https://schema.org",
          "@type": "WebPage",
          "name": "Example page"
        }
      }
    ]
  },
  "route": {
    "name": "entity.canvas_page.canonical",
    "requestUri": "/page/1",
    "params": {
      "canvas_page": "1"
    },
    "entity": {
      "entityType": "canvas_page",
      "bundle": "canvas_page",
      "id": "1",
      "uuid": "773942c6-3660-4c50-9a8d-e25966a69bff",
      "langcode": "en"
    }
  }
}

content contains one structured root when Canvas manages the complete component tree. Multiple roots use a transparent renderless-container with the ordered roots in its default slot. When Canvas does not render the matched route, content is null.

head is compatible with the Unhead library. It always contains title and may also contain meta, link, and script. Canonical links are omitted because the frontend owns its public URLs.

Redirect response

{
  "redirect": {
    "external": false,
    "url": "/new-path",
    "statusCode": 301
  }
}

Redirect results use HTTP 200. statusCode is the status the frontend should use for the browser redirect.

Error response

Route-not-found, access-denied, and invalid requests use RFC 9457 Problem Details with the application/problem+json media type:

{
  "type": "about:blank",
  "title": "Not Found",
  "status": 404
}

detail is included when an additional explanation is available, such as when requestUri is missing or invalid.

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