Interactivity API: populate router's state.url in the server

[luisherranz]

What

This PR populates state.url in the core/router Interactivity API namespace on the server, inside the data-wp-router-region directive processor.

Trac ticket: https://core.trac.wordpress.org/ticket/64649

Why

After Gutenberg PR WordPress/gutenberg#70882, state.url in the interactivity router is no longer unconditionally set to window.location.href on the client. Instead, it falls back to window.location.href only if the server hasn't already provided a value:

state.url = state.url || window.location.href;

This PR provides that server-side value by calling wp_interactivity_state( 'core/router', ... ) during directive processing, following the same URL construction pattern used by redirect_canonical() and get_self_link().

How

• Adds a wp_interactivity_state( 'core/router', array( 'url' => ... ) ) call inside data_wp_router_region_processor() in WP_Interactivity_API.
• The URL is constructed using set_url_scheme(), $_SERVER['HTTP_HOST'], and $_SERVER['REQUEST_URI'].

Testing

1. Create a page with a Query Loop block using enhanced pagination (which uses data-wp-router-region).
2. View the page source and verify the interactivity script module data JSON includes "core/router" with the "url" property matching the current page URL.
3. Verify client-side navigation still works correctly.

Use of AI tools

Claude Code was used to research the best way to construct the URL and how other internal WordPress functions do it, and assist in creating the tests.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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