Update wp_is_mobile() to account for Sec-CH-UA-Mobile request header

[westonruter]

The wp_is_mobile() function currently uses User-Agent sniffing to determine whether the client is a mobile device. There is a new more reliable way to determine this: ​Client Hints. Specifically, there is a ​Sec-CH-UA-Mobile HTTP request header which gets the value of ?1 for a mobile device and the value of ?0 for a desktop device. This header is ​supported by ~70% of browsers today. It is supported in Chrome, Edge, and Opera. It is not currently supported by Safari or Firefox. Nevertheless, if the function can just be extended to support this request header as its first case, and then fall back to the existing User-Agent sniffing:

src/wp-includes/vars.php

@@ -146 +146 @@
  * @return bool
  */
 function wp_is_mobile() {
-        if ( empty( $_SERVER['HTTP_USER_AGENT'] ) ) {
+        if ( isset( $_SERVER['HTTP_SEC_CH_UA_MOBILE'] ) ) {
+                $is_mobile = ( '?1' === $_SERVER['HTTP_SEC_CH_UA_MOBILE'] );
+        } elseif ( empty( $_SERVER['HTTP_USER_AGENT'] ) ) {
                 $is_mobile = false;
         } elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Mobile' ) // Many mobile devices (all iPhone, iPad, etc.)
                 || str_contains( $_SERVER['HTTP_USER_AGENT'], 'Android' )

Trac ticket: https://core.trac.wordpress.org/ticket/59370

[westonruter]

In 56638:

General: Account for Sec-CH-UA-Mobile client hint request header in wp_is_mobile().

Add missing test coverage for wp_is_mobile().

Fixes #59370.
Props westonruter, flixos90.

Committed in r56638