chore: Change Debug to final val in PhasedFusingActorMaterializer (#32800)

Author: He-Pin

akka-stream/src/main/scala/akka/stream/impl/PhasedFusingActorMaterializer.scala

@@ -48,9 +48,9 @@ import akka.util.OptionVal
  * INTERNAL API
  */
 @InternalApi private[akka] object PhasedFusingActorMaterializer {
-
-  val Debug = false
-  val Mailbox: String = "akka.stream.materializer.mailbox"
+  // Compile time constant
+  final val Debug = false
+  final val Mailbox = "akka.stream.materializer.mailbox"
 
   val DefaultPhase: Phase[Any] = new Phase[Any] {
     override def apply(