Fix the Book generator has a non existing publisher.

test/support/generators.ex

@@ -134,10 +134,10 @@ defmodule DecentralisedBookIndex.Generators do
           author_roles()
         end)
 
-    publisher =
-      opts[:publisher] ||
+    publisher_id =
+      opts[:publisher_id] ||
         once(:default_publisher, fn ->
-          generate(publisher())
+          generate(publisher()).id
         end)
 
     changeset_generator(
@@ -152,7 +152,7 @@ defmodule DecentralisedBookIndex.Generators do
         published: sequence(:published, &Date.add(~D[2025-03-04], &1)),
         bids: bids,
         author_roles: author_roles,
-        publisher: publisher,
+        publisher_id: publisher_id,
         book_editions_registry_id: nil,
       ],
       overrides: opts,