Fix PHPStan errors introduced by mail and certificate publisher refactor

- Make Mail and Certificate message classes final with proper typed PHPDoc
- Use @var assertions and is_string() guards at call sites instead of mixed casts
- Add ?? fallback on Queue constructor calls to resolve string|null
- Add MailsPublisher type hint to untyped closure parameter
- Regenerate phpstan baseline to remove stale old Mail setter suppressions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/controllers/api/projects.php

@@ -1863,6 +1863,7 @@ Http::post('/v1/projects/:projectId/smtp/tests')
             ->setParam('{{privacyUrl}}', $plan['privacyUrl'] ?? APP_EMAIL_PRIVACY_URL);
 
         foreach ($emails as $email) {
+            /** @var string $email */
             $publisherForMails->enqueue(new Mail(
                 project: $project,
                 recipient: $email,