Added contents to book-show endpoint

Created a generic list formatting helper class for this, to align with
logic used on the search results endpoint and for easier future re-use
in a standardised way.
Also updated some class property types.
Added test to cover new books-contents results.
Related to #3734

app/Http/Controllers/Api/BookshelfApiController.php

@@ -13,9 +13,6 @@ class BookshelfApiController extends ApiController
 {
     protected BookshelfRepo $bookshelfRepo;
 
-    /**
-     * BookshelfApiController constructor.
-     */
     public function __construct(BookshelfRepo $bookshelfRepo)
     {
         $this->bookshelfRepo = $bookshelfRepo;