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/BookApiController.php

@@ -2,14 +2,18 @@
 
 namespace BookStack\Http\Controllers\Api;
 
+use BookStack\Api\ApiEntityListFormatter;
 use BookStack\Entities\Models\Book;
+use BookStack\Entities\Models\Chapter;
+use BookStack\Entities\Models\Entity;
 use BookStack\Entities\Repos\BookRepo;
+use BookStack\Entities\Tools\BookContents;
 use Illuminate\Http\Request;
 use Illuminate\Validation\ValidationException;
 
 class BookApiController extends ApiController
 {
-    protected $bookRepo;
+    protected BookRepo $bookRepo;
 
     public function __construct(BookRepo $bookRepo)
     {
@@ -47,11 +51,25 @@ class BookApiController extends ApiController
 
     /**
      * View the details of a single book.
+     * The response data will contain 'content' property listing the chapter and pages directly within, in
+     * the same structure as you'd see within the BookStack interface when viewing a book. Top-level
+     * contents will have a 'type' property to distinguish between pages & chapters.
      */
     public function read(string $id)
     {
         $book = Book::visible()->with(['tags', 'cover', 'createdBy', 'updatedBy', 'ownedBy'])->findOrFail($id);
 
+        $contents = (new BookContents($book))->getTree(true, false)->all();
+        $contentsApiData = (new ApiEntityListFormatter($contents))
+            ->withType()
+            ->withField('pages', function (Entity $entity) {
+                if ($entity instanceof Chapter) {
+                    return (new ApiEntityListFormatter($entity->pages->all()))->format();
+                }
+                return null;
+            })->format();
+        $book->setAttribute('contents', $contentsApiData);
+
         return response()->json($book);
     }