REST API: Call `rest_get_server()` instead of accessing the `$wp_rest_server` global

[jnylen0]

It is possible, though unlikely, that REST API functions can be called from within a context that has not actually loaded a WP_REST_Server instance into the $wp_rest_server variable. This has caused a few crashes on WP.com; details are well within the nightmare zone of customized WordPress load order.

There is already a rest_get_server() function to ensure that the server object is initialized before attempting to do anything with it; we should be consistent in using this function rather than the global variable.

This was mostly done in [36529] (cc @rmccue); the patch attached here cleans up all remaining uses of $wp_rest_server except for those within the rest_get_server() function, along with a few errant @global annotations.

[jnylen0]

Add a previously failing test case

[jnylen0]

Added a unit test in 41555.2.diff​. Before these changes, it fails with the same message we saw on WP.com:

Fatal error: Call to a member function register_route() on null in src/wp-includes/rest-api.php on line 77

[rmccue]

Makes sense to me.

[jnylen0]

Fix PHPUnit setUp and tearDown calls

[jnylen0]

In 41238:

REST API: Always call rest_get_server() instead of accessing the $wp_rest_server global.

This is a consistency improvement and also a bug fix for fairly obscure cases involving modified WP load order.

Fixes #41555.