Add default_scope to BookComment for stable ordering

The previous fix adding order(:id) to Book's has_many :book_comments
association was not sufficient. In some contexts (particularly with
Rails 8 and caching), the association scope was not being applied
consistently, leading to non-deterministic ordering in PostgreSQL.

Adding default_scope { order(:id) } to BookComment ensures that all
queries against this model return results in a consistent order,
regardless of how they are accessed (direct queries, associations,
eager loading, etc.).

This is a test-only change that ensures stable test results across
different Rails versions and database engines.

Fixes: RequestTest#test_pagination_related_resources_data_includes
       (flaky on Rails 8.1.2 + PostgreSQL)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: takaokouji

test/fixtures/active_record.rb

@@ -658,6 +658,8 @@ class Book < ActiveRecord::Base
 end
 
 class BookComment < ActiveRecord::Base
+  default_scope { order(:id) }
+
   belongs_to :author, class_name: 'Person', foreign_key: 'author_id'
   belongs_to :book