Interactivity API: Initialize router's `state.url` on the server

[luisherranz]

The Interactivity API router currently initializes state.url on the client using window.location.href. With the changes in Gutenberg PR ​#70882, this initialization was moved to a fallback pattern (state.url = state.url || window.location.href) so that the server can populate it first.

This ticket adds the server-side initialization of state.url in the core/router namespace. The URL is set during the processing of the data-wp-router-region directive, using the same pattern that WordPress core uses internally to construct the current request URL (see redirect_canonical() and get_self_link()):

wp_interactivity_state(
    'core/router',
    array(
        'url' => set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . wp_unslash( $_SERVER['REQUEST_URI'] ) ),
    )
);

This enables proper server-side rendering of directives that reference state.url in the core/router store.

### 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 ​https://github.com/WordPress/gutenberg/pull/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.

[luisherranz]

In 61659:

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

Set the core/router state url property with the current page URL using get_self_link() when the first data-wp-router-region directive is processed, so that the stores subscribing to that property already have the correct value on page load.

Props westonruter.
Fixes #64649.

Thanks (as always), Weston! Committed 🙏