Use lower-case toString for Visibility

The `toString()` value is used to generate the api documentation. To
make the docs match the actual result values, `toString()` is mapped
to `name` for `Visibility`.

Fixes: #160

modules/commons/src/main/scala/io/renku/search/model/Visibility.scala

@@ -27,6 +27,8 @@ enum Visibility:
   lazy val name: String = productPrefix.toLowerCase
   case Public, Private
 
+  override def toString(): String = name
+
 object Visibility:
   given Order[Visibility] = Order.by(_.ordinal)
   given Encoder[Visibility] = Encoder.forString.contramap(_.name)