Fix authenticated user must implement CraftUser error

[brianjhanson]

Whilst setting up a new Craft 6 instance I ran into a The authenticated user must implement CraftCms\Cms\User\Contracts\CraftUser to be used by Craft. error when trying to log in after installing Craft via the CLI (artisan craft:install).

This fixes the issue.

One question Claud had that I'm not sure of the answer to:

Do you want the framework to keep silently taking over the provider like this, or would you prefer it to log/warn when it overrides a mismatched model?

I'm not 100% sure of our intent here, so this fix may not be 100% correct but here's the explanation from Claude:

---

AuthServiceProvider::register() only re-pointed the auth provider at Craft's User model when the configured model class didn't exist:

if (! class_exists($this->app->make(Repository::class)->get('auth.providers.users.model'))) {
    $this->app->make(Repository::class)->set('auth.providers.users.model', User::class);
}

A standard Laravel app ships with App\Models\User, which does exist but does not implement CraftCms\Cms\User\Contracts\CraftUser. Because the class existed, Craft skipped the override, so Auth::user() returned a plain App\Models\User. On the next request, currentUser() (src/helpers.php:91) rejected the non-Craft user and threw an AuthenticationException, surfacing as a 500 during CP login.

The guard's original intent was to avoid clobbering a developer's custom user model — but a Craft user model is required to implement CraftUser, so "the class exists" was the wrong condition.

### Fix

This fix checks whether the configured model actually implements CraftUser, rather than merely whether the class exists

[riasvdv]

Changing this would lead to unexpected behavior for users as we’d be changing which user model is used.

I think the reason you got that exception is because you didn’t get the version of craftcms/craft yet with this change: craftcms/craft@9004c03