fix: Add content_id to inline attachment handling in CloudflareTransport (#60330)

CloudFlare requires the `content_id` key to be set for inline attachments.
See docs: https://developers.cloudflare.com/api/resources/email_sending/methods/send/

If missing, it fails with Symfony\Component\Mailer\Exception\TransportException: email.sending.error.invalid_request_schema

This solution uses the same idea as src/Illuminate/Mail/Transport/ResendTransport.php:94 which also requires `content_id`.

Besides the test, I have also manually sent an email with an inline image and confirmed the image is shown in the email I received.