Add support for ui_mode: custom in Stripe Checkout Sessions

[emaadali]

Currently, Cashier treats ui_mode: custom the same as hosted mode, causing it to incorrectly require success_url/cancel_url, and generate errors such as:

• Route [home] not defined.
• The following parameters are not supported with 'ui_mode: custom': cancel_url, success_url.
• See UI mode 'custom' is not working when creating checkout session #1780

This PR ensures custom sessions are handled the same way as embedded sessions and return the Stripe session object as expected:

- if (isset($data['ui_mode']) && $data['ui_mode'] === 'embedded') {
+ if (isset($data['ui_mode']) && in_array($data['ui_mode'], ['embedded', 'custom'])) {
    $data['return_url'] = $sessionOptions['return_url'] ?? route('home');

    // Remove return URL for embedded UI mode when no redirection is desired on completion...
    if (isset($data['redirect_on_completion']) && $data['redirect_on_completion'] === 'never') {
        unset($data['return_url']);
    }
} else {
    $data['success_url'] = $sessionOptions['success_url'] ?? route('home').'?checkout=success';
    $data['cancel_url'] = $sessionOptions['cancel_url'] ?? route('home').'?checkout=cancelled';
}

[emaadali]

@taylorotwell This merge closes #1780