fix(mail): install the 4 mailer bridges the code already builds DSNs for (#450)

SymfonyEmailService builds Transport::fromDsn() URLs for five API providers,
but only one bridge was installed. The other four threw
UnsupportedSchemeException at SEND time — configuration accepted, mail lost.

  line 369  sendgrid+api://   symfony/sendgrid-mailer   MISSING
  line 390  mailgun+api://    symfony/mailgun-mailer    MISSING
  line 411  postmark+api://   symfony/postmark-mailer   MISSING
  line 433  ses+api://        symfony/amazon-mailer     MISSING
  line 459  mailjet+api://    symfony/mailjet-mailer    present
  line 331  smtp://           symfony/mailer            present

This is the inverse of the usual dependency problem: not an unused package,
but a MISSING require behind code that already exists and is reachable from
the settings UI. Anyone selecting SendGrid, Mailgun, Postmark or SES got a
working-looking configuration and no mail.

Verified after installing, against the real autoloader:

  sendgrid  RESOLVES
  mailgun   RESOLVES
  postmark  RESOLVES
  ses       RESOLVES
  mailjet   RESOLVES

And the check is discriminating, not vacuous — schemes whose bridges are
deliberately absent still report unsupported:

  brevo     UNSUPPORTED
  infobip   UNSUPPORTED

All four bridges pinned ^6.4 to match symfony/mailer. composer audit clean.

Co-authored-by: Conduction Release Bot <release-bot@conduction.nl>

Author: rubenvdlinde

composer.json

@@ -69,9 +69,13 @@
 		"bamarni/composer-bin-plugin": "^1.8",
 		"react/event-loop": "^1.5",
 		"react/promise": "^3.2",
+		"symfony/amazon-mailer": "^6.4",
 		"symfony/http-client": "^6.4",
 		"symfony/mailer": "^6.4",
+		"symfony/mailgun-mailer": "^6.4",
 		"symfony/mailjet-mailer": "^6.4",
+		"symfony/postmark-mailer": "^6.4",
+		"symfony/sendgrid-mailer": "^6.4",
 		"twig/twig": "^3.27.0"
 	},
 	"require-dev": {