fix(hysteria2): emit https server_url for sing-box realm outbound (#9642)

sing-box requires realm.server_url as a full URL (https://host:port).
Bare host:port caused "missing host in realm server_url" on import.
Also forward stun_servers and clear ResolveRealm success message.

Fixes #9635

v2rayN/ServiceLib/Handler/Fmt/Hysteria2Fmt.cs

@@ -110,6 +110,7 @@ public class Hysteria2Fmt : BaseFmt
             Hy2RealmUrl = realm.ToUri(),
         });
 
+        msg = string.Empty;
         return item;
     }