Fix unstable ordering in Book associations for PostgreSQL

Add explicit ordering by id to Book's book_comments and
approved_book_comments associations. Without explicit ordering,
PostgreSQL returns relationship data in non-deterministic order,
causing cache test failures where warmup response differs from
normal response.

The previous fix added default_sort to BookCommentResource, which
handles collection queries, but didn't affect relationship linkage
data in included resources. This fix ensures stable ordering at
the ActiveRecord association level.

Fixes: RequestTest#test_pagination_related_resources_data_includes

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

Author: takaokouji

test/fixtures/active_record.rb

@@ -647,8 +647,8 @@ def destroy
 end
 
 class Book < ActiveRecord::Base
-  has_many :book_comments
-  has_many :approved_book_comments, -> { where(approved: true) }, class_name: "BookComment"
+  has_many :book_comments, -> { order(:id) }
+  has_many :approved_book_comments, -> { where(approved: true).order(:id) }, class_name: "BookComment"
 
   has_and_belongs_to_many :authors, join_table: :book_authors, class_name: "Person"