fox: use Mercure public URL for subscription URI creation if it is available (#6514)

This is required when publishing and reading host names are not the same, ex: docker environment ("mercure" vs "localhost:port" between server side and client side), having different publishing URL and public access url's, etc.
Mercure itself and Mercure bundle have been supporting this for a while.

Author: psihius

src/GraphQl/Subscription/MercureSubscriptionIriGenerator.php

@@ -41,6 +41,6 @@ public function generateTopicIri(string $subscriptionId): string
 
     public function generateMercureUrl(string $subscriptionId, ?string $hub = null): string
     {
-        return \sprintf('%s?topic=%s', $this->registry->getHub($hub)->getUrl(), $this->generateTopicIri($subscriptionId));
+        return \sprintf('%s?topic=%s', $this->registry->getHub($hub)->getPublicUrl(), $this->generateTopicIri($subscriptionId));
     }
 }