[12.x] Make Fluent class iterable

[xurshudyan]

Currently, while Fluent supports array-style access (ArrayAccess) and dynamic property access, it cannot be directly iterated. This gap often causes inconvenience in Laravel applications, especially when working with Eloquent attribute casting such as:

protected function casts(): array
{
    return [
        'settings' => \Illuminate\Database\Eloquent\Casts\AsFluent::class
    ];
}

Without this enhancement, developers must call ->toArray() before iterating over casted attributes

foreach ($user->settings->toArray() as $key => $value) {
    // 
}

By enabling native iteration, this change simplifies and cleans up common usage patterns, allowing

foreach ($user->settings as $key => $value) {
    //
}