Add X-Drupal-CE response header to identify CE-API responses

Problem

There is no way for consumers (e.g. Nuxt frontend) to distinguish a proper Custom Elements API response from other JSON responses served via /ce-api. Routes like /api/menu_items/main return application/json just like CE page responses do. When the frontend's catch-all page handler receives a non-CE JSON response, it breaks (500) because it expects the CE page structure (title, content, metatags, etc.).

Proposed resolution

Add an X-Drupal-Ce response header to all CustomElementsJsonResponse responses:

- X-Drupal-CE: page — for regular CE page responses
- X-Drupal-CE: redirect — for redirect responses converted to JSON

This is BC-compatible — no existing behavior changes, consumers can opt-in to checking the header.

Consumer side (nuxtjs-drupal-ce)

With the header available, the Nuxt server-side proxy (drupalCe.ts) could inspect the response header and, when X-Drupal-Ce is absent, either proxy the raw response through or redirect to the backend, instead of letting the page handler crash on unexpected JSON.