Strip <link rel="alternate"> from global header REST output

[Copilot]

The global header REST endpoint includes wp_head() output for styles, which also emits <link rel="alternate"> tags (feed links, oEmbed discovery, hreflang from plugins, etc.) that are irrelevant when the header is embedded in other sites. The previous approach used remove_action on known core hooks, which missed tags added by plugins.

• Replaced remove_action calls (feed_links, feed_links_extra, wp_oembed_add_discovery_links) with a regex-based strip on the rendered $head_markup, catching all sources
• Added remove_head_alternate_links() — strips <link> tags where rel contains the alternate token, applied only during REST requests

function remove_head_alternate_links( $markup ) {
	return preg_replace( '/<link\b[^>]*\brel=["\'][^"\']*\balternate\b[^"\']*["\'][^>]*\/?>/i', '', $markup );
}

Word boundary (\b) prevents false matches on values like rel="myalternate".

• Added PHPUnit tests (phpunit/test-global-header-footer.php) covering removal of RSS/Atom feed links, oEmbed discovery, hreflang tags, single-quoted and non-self-closing variants, compound rel values, preservation of non-alternate links (stylesheet, canonical, preconnect, icon), mixed markup, and edge cases.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Don't output <link rel="alternate" in the headers</issue_title>
<issue_description>In https://meta.trac.wordpress.org/changeset/14677 I removed the <link rel="alternate" tags from the Trac templates, but doesn't remove it from other things (404/403/codex/etc)

The Global Header output however shouldn't have these tags at all.

We remove the feed links, but we don't remove the tags added by plugins.

We should use WP_HTML_Tag_Parser to remove these from the output.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Don't output <link rel="alternate" in the headers #733

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[dd32]

@copilot Can you add any tests for this?

[Copilot]

@copilot Can you add any tests for this?

Added PHPUnit tests in 06421af. The test file covers: removing RSS/Atom feed links, oEmbed discovery, hreflang tags, single-quoted and non-self-closing variants, compound rel values, and preserving non-alternate links (stylesheet, canonical, preconnect, icon). Also tests mixed markup and edge cases (empty input, no link tags).