Symfony version(s) affected

6.1.*

Description

When using der MailJet Mailer component using the MailJet API in conjunction with template variables, the MailJet component only allows for a max depth of two (2), thus we're getting the following error while trying to send emails:

JsonException
in vendor/symfony/mailjet-mailer/Transport/MailjetApiTransport.php (line 204)

199    private function castCustomHeader(string $value, string $type)
200   {
201        return match ($type) {
202            'bool' => filter_var($value, \FILTER_VALIDATE_BOOLEAN),
203            'int' => (int) $value,
204            'json' => json_decode($value, true, 2, \JSON_THROW_ON_ERROR), <--
205            'string' => $value,
206       };
207    }
208 }

When using the MailJet PHP API (https://github.com/mailjet/mailjet-apiv3-php) there's no limitation to the depth of the variable template payload.

How to reproduce

1. Create a free MailJet account for using the MailJet API
2. Create a new Symfony app
3. Require both symfony/mailer and symfony/mailjet-mailer using composer
4. Configure the MAILER_DSN to use the MailJet API transport as described in the documentation https://symfony.com/doc/current/mailer.html#using-a-3rd-party-transport
5. Try to send an email that uses MailJet templates and add a multi-dimensional array for template variables

Example:

$templateId = 1337;

$templateVars = [
    'foo' => [
        'bar' => 'baz',
    ],
];

$email = (new Symfony\Component\Mime\Email())
    ->from('foo@localhost')
    ->to('bar@localhost')
    ->subject('Test email');
    
$email->text('Ignored, because using MailJet templates');

$email
    ->getHeaders()
    ->addTextHeader('X-MJ-TemplateLanguage', 'true')
    ->addTextHeader('X-MJ-TemplateID', $templateId)
    ->addTextHeader('X-MJ-Vars', \json_encode($templateVars));
    
$this->mailer->send($email);

Possible Solution

Remove the limitation that allows for only a max depth of 2 levels here: https://github.com/symfony/mailjet-mailer/blob/6.1/Transport/MailjetApiTransport.php#L204

Additional Context

No response