Timout await extension initialization, #28301 (#28340)

* use the existing creation-timeout config
* and error message about the typical mistake of accessing
  SerializationExtension from constructor of serializer
* mention in docs

akka-docs/src/main/paradox/serialization.md

@@ -127,6 +127,10 @@ bytes to different objects.
 Then you only need to fill in the blanks, bind it to a name in your configuration and then
 list which classes that should be serialized using it.
 
+The serializers are initialized eagerly by the `SerializationExtension` when the `ActorSystem` is started and
+therefore a serializer itself must not access the `SerializationExtension` from its constructor. Instead, it
+should access the `SerializationExtension` lazily.
+
 <a id="string-manifest-serializer"></a>
 ### Serializer with String Manifest
 

akka-docs/src/test/java/jdocs/serialization/SerializationDocTest.java

@@ -196,8 +196,7 @@ public class SerializationDocTest {
         akka.actor.typed.ActorSystem.create(Behaviors.empty(), "example");
 
     // Get the Serialization Extension
-    Serialization serialization =
-        SerializationExtension.get(akka.actor.typed.javadsl.Adapter.toClassic(system));
+    Serialization serialization = SerializationExtension.get(system);
     // #programmatic-typed
   }
 }

akka-docs/src/test/scala/docs/serialization/SerializationDocSpec.scala

@@ -214,12 +214,11 @@ package docs.serialization {
     def demonstrateTypedActorSystem(): Unit = {
       //#programmatic-typed
       import akka.actor.typed.ActorSystem
-      import akka.actor.typed.scaladsl.adapter._
 
       val system = ActorSystem(Behaviors.empty, "example")
 
       // Get the Serialization Extension
-      val serialization = SerializationExtension(system.toClassic)
+      val serialization = SerializationExtension(system)
       //#programmatic-typed
     }