RFC 9110 Accept headers, dotenv fixes, and Ractor safety

This week in Rails covers a batch of bug fixes and improvements across the framework. Key changes include an opt-in for RFC 9110 compliant Accept header content negotiation, dotenv fixes for inline comment stripping and path memoization, several Active Record fixes around composite primary keys, polymorphic associations, and create_or_find_by scope pollution. Other fixes address mutation of caller-owned state in url_for, Http::Headers#merge, and ActiveModel::Errors#import. Ractor safety work continues with frozen defaults and copy-on-write patterns applied to TimeFormats, rescue handlers, parameter encodings, view cache dependencies, and more.