Build 'shared_navigation' schema and 'navigation_items' reverse link with expandable members [WHIT-3342]

[ChrisBAshton]

What

• Defines a new shared_navigation content type
• Defines a new shared_navigations->navigation_items reverse link
• Configures the shared_navigation content type to have its details.menu_items hash included when it is surfaced in link expansion
• Configures a new REVERSE_LINKS_WITH_MEMBER_EXPANSION feature for Publishing API in which all members of a link are also expanded. See below.

REVERSE_LINKS_WITH_MEMBER_EXPANSION

When running DownstreamPayload.new(Document.find_by(content_id: press_release_content_id, locale: "en").editions.last, 1, draft: true).expanded_links.

Before - only the Press Release content item itself would be expanded in shared_navigations[n].links:

{shared_navigations:
  [{content_id: "81392be0-5ccc-4a84-8431-79f1920b7242",
    title: "Chris shared navigation June",
    locale: "en",
    analytics_identifier: nil,
    api_path: "/api/content/chris-shared-navigation-june--2",
    base_path: "/chris-shared-navigation-june--2",
    document_type: "shared_navigation",
    public_updated_at: "2026-06-02T14:58:28Z",
    schema_name: "shared_navigation",
    withdrawn: false,
    details: {menu_items: [{type: "press_release", content_id: "26086fc1-fbd6-4291-b308-3f334d380d56"}, {type: "publication", content_id: "c6caa8d4-5557-42ea-935b-d45d640d4269"}]},
    links:
     {navigation_items:
       [{content_id: "26086fc1-fbd6-4291-b308-3f334d380d56",
         title: "Foreign Secretary statement on the situation in Lebanon",
         locale: "en",
         analytics_identifier: nil,
         api_path: "/api/content/government/news/foreign-secretary-statement-on-the-situation-in-lebanon--2",
         base_path: "/government/news/foreign-secretary-statement-on-the-situation-in-lebanon--2",
         document_type: "press_release",
         public_updated_at: "2026-03-15T23:02:00Z",
         schema_name: "news_article",
         withdrawn: false,
         links: {}}]}}],

After - ALL of the documents linked to the shared navigation are now expanded in shared_navigations[n].links:

{shared_navigations:
  [{content_id: "81392be0-5ccc-4a84-8431-79f1920b7242",
    title: "Chris shared navigation June",
    locale: "en",
    analytics_identifier: nil,
    api_path: "/api/content/chris-shared-navigation-june--2",
    base_path: "/chris-shared-navigation-june--2",
    document_type: "shared_navigation",
    public_updated_at: "2026-06-02T14:58:28Z",
    schema_name: "shared_navigation",
    withdrawn: false,
    details: {menu_items: [{type: "press_release", content_id: "26086fc1-fbd6-4291-b308-3f334d380d56"}, {type: "publication", content_id: "c6caa8d4-5557-42ea-935b-d45d640d4269"}]},
    links:
     {navigation_items:
       [{content_id: "26086fc1-fbd6-4291-b308-3f334d380d56",
         title: "Foreign Secretary statement on the situation in Lebanon",
         locale: "en",
         analytics_identifier: nil,
         api_path: "/api/content/government/news/foreign-secretary-statement-on-the-situation-in-lebanon--2",
         base_path: "/government/news/foreign-secretary-statement-on-the-situation-in-lebanon--2",
         document_type: "press_release",
         public_updated_at: "2026-03-15T23:02:00Z",
         schema_name: "news_article",
         withdrawn: false,
         links: {}},
        {content_id: "c6caa8d4-5557-42ea-935b-d45d640d4269",
         title: "Youth justice plans: guidance for youth justice services",
         locale: "en",
         analytics_identifier: nil,
         api_path: "/api/content/government/publications/youth-justice-plans-guidance-for-youth-justice-services",
         base_path: "/government/publications/youth-justice-plans-guidance-for-youth-justice-services",
         document_type: "guidance",
         public_updated_at: "2025-01-21T14:33:30Z",
         schema_name: "publication",
         withdrawn: false,
         links: {}}]}}],

This is essential for developing the new "shared navigation" feature for landing pages in Q2, as these links define the content of the navigation. The shared_navigation content item itself will only have the structure of the navigation in its details hash.

(The latter is currently an arbitrary hash, but the idea is it would contain metadata such as whether a given navigation item should be hidden/shown/highlighted, its order, which nav items are nested within it, etc. The Publishing API schema for it has been kept deliberately permissive for now while we iron out those requirements.)

Why

In order to support the new "shared navigation" feature for landing pages, a key deliverable of Q2.

Jira: https://gov-uk.atlassian.net/browse/WHIT-3342

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