Boost: Make sure Critical CSS always works with absolute URLs

[dilirity]

Partially addresses #39455.

Proposed changes:

• Prevent source providers from returning relative URLs. Bad for Critical/Cloud CSS.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1726628647463949-slack-C016BBAFHHS

Does this pull request change what data or activity we track or use?

no

Testing instructions:

To see the critical css source-providers endpoint (wp-json/jetpack-boost/v1/list-source-providers), you'll need to have the Boost Developer plugin running.

Test that it shows relative URLs

• Setup Boost free from trunk;
• Add this snippet to your site - this will make posts have relative URLs (make sure you have at least one post on your site):

add_filter('post_link', 'make_post_url_relative', 10, 2);
function make_post_url_relative($url, $post_id) {
    if (get_post_type($post_id) === 'post') {
        return wp_make_link_relative($url);
    }
    return $url;
}

• Visit the endpoint URL - http://jetpack-boost.test/wp-json/jetpack-boost/v1/list-source-providers
• There should be relative URLs on the page;
• Try to generate Critical CSS - it should fail with a fatal error similar to the one below:

Test that it doesn't show relative URLs

• Switch to this PR's branch;
• Visit the endpoint URL again;
• There should be no relative URLs;
• Try to generate Critical CSS - it should work.

[github-actions]

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[donnchawp]

Works as expected, even when there's only one URL, it's passed as an array of one.