No support for OAuth2 with `client_credentials`

So I tried both versions 3.1.0 and 4.0.0-beta, and it seems this library completely omits support for grantType with client_credentials, do I understand that correctly?

(PS. version 3.1.0 - there's a fatal error when doing a fresh composer install because of an interface mismatch with the psr package, so the current published version is unusable.)

So it seems, using OAuth2, this library is incapable of just doing a simple POST request to the server and capturing an access_token. It tries to redirect me between pages and asks me to log in as a user...?

I was trying to use this Mautic API Library to communicate between my web application and my self-hosted Mautic server, and so far I've spent 3 hours trying to get a basic connection going with OAuth2.

I've deep-dived through the code for generating an access_token, and it doesn't seem to make any sense, there's no mention of client_credentials anywhere in the OAuth code.

Here's some simple example code to test the OAuth2:

use Mautic\MauticApi;
use Mautic\Auth\ApiAuth;

$baseUrl = 'https://my-mautic-server.com';
$publicKey = '1234567890';
$secretKey = '1234567890';

$settings = [
    'baseUrl' => $baseUrl,
    'version' => 'OAuth2',
    'clientKey' => $publicKey,
    'clientSecret' => $secretKey,
    'grantType' => 'client_credentials',
];

$initAuth = new ApiAuth();

/** @var Mautic\Auth\OAuth $auth */
$auth = $initAuth->newAuth($settings);

if ( $auth->validateAccessToken(false) ) {
    //yay, it worked!
} else {
    throw new Exception('Failed to authenticate with Mautic API.');
}

$client = new MauticApi();
$segmentsApi = $client->newApi('segments', $auth, $baseUrl);
$results = $segmentsApi->getList();

var_dump("Results: ", $results);

The result is always a 401 unauthenticated:

{"errors":[{"code":401,"message":"The response has unexpected status code (401).\n\nResponse: {\"errors\":[{\"message\":\"API authorization denied.\",\"code\":401,\"type\":\"access_denied\"}]}"}]}

Using CURL in terminal, I'm able to generate an access token using client_credentials without any issues.

Start by reading ApiAuth, the OAuth authentication path returned by newAuth($settings), and validateAccessToken(false), using the provided client_credentials example as the reproduction case. Confirm the flow can obtain and use an access_token for the segments API without user redirects, and verify the 401 response is resolved.