Fix unstable ordering in BookCommentResource for PostgreSQL

Add default_sort to Api::V2::BookCommentResource to ensure stable
ordering across cache and non-cache scenarios. Without explicit
ordering, PostgreSQL may return results in different orders, causing
cache test failures.

Fixes: RequestTest#test_pagination_related_resources_data_includes

Author: takaokouji

test/fixtures/active_record.rb

@@ -2022,6 +2022,11 @@ class BookCommentResource < JSONAPI::Resource
         records.where(BookComment.arel_table[:body].matches("%#{value[0]}%"))
       }
 
+      # Add default sort to ensure stable ordering across cache/non-cache scenarios
+      def self.default_sort
+        [{field: 'id', direction: :asc}]
+      end
+
       class << self
         def book_comments
           BookComment.arel_table