[Mailer] Ability to identify transport by name instead of transport as string

[clement-michelet]

Description

As a developer
I want to be able to identify the transport by the transport key/name used to send an email in my test
In order to ensure the mail was sent with the selected transport

Example

Given the following mailer configuration with multiple transports:

framework:
    mailer:
        transports:
            sendmail: '%env(MAILER_SENDMAIL_DSN)%'
            brevo: '%env(MAILER_BREVO_DSN)%'
            other_brevo: '%env(MAILER_OTHER_BREVO_DSN)%'

when@prod:
  parameters:
    env(MAILER_SENDMAIL_DSN): 'sendmail://default'
    env(MAILER_BREVO_DSN): 'brevo+api://KEY@default'
    env(MAILER_OTHER_BREVO_DSN): 'brevo+api://OTHER_KEY@default'

when@test:
  parameters:
    env(MAILER_SENDMAIL_DSN): 'null://null'
    env(MAILER_BREVO_DSN): 'null://null'
    env(MAILER_OTHER_BREVO_DSN): 'null://null'

I have some code that will set the header X-Transport with the target transport to use :

$email = new Email();
$email->getHeaders()
    ->addTextHeader('X-Transport', 'brevo')
;

// [...] Set from/to/subject/body/...

$mailer->send($email);

As of now, I cannot write a functional test to test that the expected transport is used. The transport is not identified by the declared transport key (i.e sendmail, brevo, other_brevo) but instead the transport in events is the transport casted as a string.

self::assertEmailCount(1, 'brevo'); // fails
self::assertEmailCount(1, 'null://'); // pass but is it brevo or sendmail  or other_brevo as both use null transport in test env ?
self::assertEmailCount(1, 'brevo+api://api.brevo.com'); // pass if I use brevo transport but it will call the API and I don't know if it's brevo or other_brevo

As X-Transport is removed from the message, I cannot assert on the header.