IBX-12664: Added siteaccess-aware headless mode configuration

[tbialcz]

🎫 Issue	IBX-12664

Related PRs:

• https://github.com/ibexa/page-builder/pull/630 (uses the new contract)
• https://github.com/ibexa/fieldtype-page/pull/234 (removes the per-field setting)

Description:

Adds one shared, siteaccess-aware configuration for headless mode (the site is rendered by an external front-end app). Until now it lived in page-builder as page_builder.headless.*, but content preview, content view and content tree will need the same settings, so it moves here.

ibexa:
    system:
        admin_group:
            headless:
                enabled: true
                page_builder:
                    preview_url: 'http://localhost:5173/preview'   # loaded into the Page Builder iframe
                content:
                    preview_url: ~                                 # for content previews, no consumer yet
                configuration_url: ~                               # optional, SaaS links its front-end settings page here

• New config parser Headless with defaults enabled: false and all URLs null.
• New HeadlessConfigurationInterface (isEnabled(), getPageBuilderPreviewUrl(), getContentPreviewUrl(), getConfigurationUrl()), implemented by HeadlessConfiguration on top of the config resolver.
• A key omitted on a siteaccess inherits the group value. isEnabled() is just the flag, consumers also check the URL they need.

For QA:

1. Add the YAML above and clear the cache. bin/console debug:container --parameter=ibexa.site_access.config.admin_group.headless.enabled shows true, ...headless.page_builder.preview_url shows the URL.
2. Add ibexa.system.admin.headless.enabled: false. The admin siteaccess resolves enabled = false but still inherits page_builder.preview_url from the group.
3. With page-builder#630 installed: edit a landing page on the admin siteaccess, the headless Page Builder opens with the preview URL in the iframe.

Documentation:

New siteaccess configuration node headless (keys above). It replaces the unreleased page_builder.headless.enabled / base_url from page-builder#550.

[konradoboza]

I see you use that in other packages - then I would say it should be an internal contract (mark as @internal for now, we will move that to a separate namespace once Deptrac changes land in AdminUI).

[konradoboza]

Why isn't this config part of Page Builder or Page Fieldtype package?

[tbialcz]

Why isn't this config part of Page Builder or Page Fieldtype package?

I think admin-ui is the right place because this is not only for Page Builder. The same front-end URL will also be used for content previews, and the Headless edition doesn’t have Page packages. Admin-ui is the common place where everyone can access it.

[konradoboza]

+1 once #2109 (comment) is done. Reasoning: #2109 (comment).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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