Send a test mail from the admin and log why delivery failed

Form submissions marked as not sent gave no reason: the transport
exception was swallowed. Every send now goes through MailCheck, which
logs the server's answer and a hint in German to var/log/mail.log
(server lines only, no credentials). The system screen shows the mail
route, sends a test mail to any address and lists recent attempts
(ADR 121).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

config/packages/test/mailer.yaml

@@ -2,3 +2,8 @@ framework:
     mailer:
         # Tests never send real mail; the transport collects messages in memory instead.
         dsn: 'null://null'
+
+parameters:
+    # Das Versandprotokoll der Tests gehört nicht nach var/log der echten Installation.
+    kant.mail_log: '%kernel.cache_dir%/mail.log'
+    kant.mailer_dsn: 'null://null'