Test failure cause:

In WP 6.6.1 (current on-dev version used in testing), the prepare_item_for_response method had

if ( in_array( 'roles', $fields, true ) ) {
    // Defensively call array_values() to ensure an array is returned.
    $data['roles'] = array_values( $user->roles );
}

But after updating in WP 6.9, a role guard was added:

if ( in_array( 'roles', $fields, true ) && ( current_user_can( 'list_users' ) || current_user_can( 'edit_user', $user->ID ) ) ) {
    // Defensively call array_values() to ensure an array is returned.
    $data['roles'] = array_values( $user->roles );
}

This was part of a security patch, changeset 60814.

Agree we don't need to rush this through. Opened #181 to break out the discussion of the cap check change and how we want to handle it. Looking for design input from longer-tenured contributors.

From @johnbillion (emphasis added):

[the need to verify that editors can create and assign co-authors is] why those tests use an editor already. I agree, let's get rid of that assertion on the roles element in the response

Done — reverted the prior change that switched the test case to use an admin, and instead removed the assertion about the GUEST_ROLE.