[Caliban] Use a function instead of effect for greet (#311)

Author: kyri-petrou

graphql/caliban/src/main/scala/Api.scala

@@ -9,7 +9,7 @@ object ServiceSchema extends SchemaDerivation[Service]
 
 case class Query(
     posts: RIO[Service, List[Post]],
-    greet: UIO[String] = ZIO.succeed("Hello World!")
+    greet: () => String = () => "Hello World!"
 ) derives ServiceSchema.SemiAuto
 
 case class User(