Admin: Use absolute URLs in admin menu links

[ArkaPrabhaChowdhury]

Trac ticket: https://core.trac.wordpress.org/ticket/56302

This updates admin menu rendering so generated menu item href values use absolute URLs in the current admin context, while preserving already absolute/external URLs.

Validation:

• php -l src\wp-admin\menu-header.php
• php -l tests\phpunit\tests\admin\menuHeader.php
• php vendor\squizlabs\php_codesniffer\bin\phpcbf --standard=phpcs.xml.dist src\wp-admin\menu-header.php tests\phpunit\tests\admin\menuHeader.php
• php vendor\squizlabs\php_codesniffer\bin\phpcs --standard=phpcs.xml.dist src\wp-admin\menu-header.php tests\phpunit\tests\admin\menuHeader.php
• git diff --check

Note: Focused PHPUnit could not run locally because the configured test DB host \mysql\ is unavailable and Docker Desktop is not running on this machine.

[github-actions]

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[github-actions]

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