Social Links: Don't prepend URL fragments

[Mamaduka]

What?

This is a follow-up to #42167.
PR of #55543 (comment).

PR updates logic in Social Links rendered and avoids prepending URL fragments with https://.

Why?

The fragments are valid URLs often used as placeholders for Social Icons. They can also refer to social sections on a website.

Testing Instructions

1. Open a Post or Page
2. Insert the example block (snippet below).
3. Save and view the post.
4. On the front end, inspect block elements and confirm that the Instagram fragment has no https:// prefix.
5. Confirm that other links are rendered correctly.

Social Links example

<!-- wp:social-links -->
<ul class="wp-block-social-links"><!-- wp:social-link {"url":"//twitter.com/WordPress","service":"twitter"} /-->
<!-- wp:social-link {"url":"profiles.wordpress.org/wordpress/","service":"wordpress"} /-->
<!-- wp:social-link {"url":"http://github.com/WordPress","service":"github"} /-->
<!-- wp:social-link {"url":"#instagram","service":"instagram"} /--></ul>
<!-- /wp:social-links -->

Testing Instructions for Keyboard

Same.

Screenshots or screencast

[github-actions]

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Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[github-actions]

Flaky tests detected in 7d7c7f4.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12764706932
📝 Reported issues:

• [Flaky Test] should show the original content after revert and page reload #68604 in /test/e2e/specs/site-editor/template-revert.spec.js
• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js