fix(NotificationGenerator): catch InvalidArgumentException from notification setters in prepare()

INotification setters like setRichSubject() and setParsedSubject() throw
\InvalidArgumentException for empty/invalid values, which previously escaped
prepare() — triggering the NC39 deprecation. Catch and discard the notification
as already-processed since it cannot be rendered.

AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

lib/NotificationGenerator.php

@@ -118,7 +118,12 @@ public function prepare(INotification $notification, string $languageCode): INot
 		$event = $this->populateEvent($event, $languageCode);
 		$this->activityManager->setCurrentUserId(null);
 
-		return $this->getDisplayNotificationForEvent($event, $event->getObjectId());
+		try {
+			return $this->getDisplayNotificationForEvent($event, $event->getObjectId());
+		} catch (\InvalidArgumentException $e) {
+			$this->logger->error('Failed to format activity notification', ['exception' => $e]);
+			throw new AlreadyProcessedException();
+		}
 	}
 
 	private function getDisplayNotificationForEvent(IEvent $event, int $activityId): INotification {