Enable the Rails 7.2 framework defaults

Why

Separate from the framework bump because it is a deploy-ordering requirement, not review hygiene. Some framework defaults change the on-disk format of cached values and signed messages — cache_format_version, message_serializer and marshalling_format_version all did in 7.1, and the existing new_framework_defaults_7_1.rb carries the team's own "Does rolling deploy apply here?" annotations. A format flip must not ship in the same deploy as the framework bump, or old and new processes cannot read each other's data during the rollout.

Scope

• Uncomment the options in new_framework_defaults_7_2.rb one group at a time
• Bump config.load_defaults to 7.2 and delete the file once all are enabled
• config.active_support.cache_format_version -> 7.2 only after the 7.2 code is fully deployed

For each option, check whether it affects a serialization format. If none of them do, this can be folded back into the Rails 7.2 issue — check rather than assume.

Acceptance criteria

• Full suite green
• load_defaults 7.2, defaults file deleted
• Deployed separately from the framework bump

Blocked by

#883 (Rails 7.2), deployed and soaked. Blocks #886.

Part of the Ruby + Rails upgrade sequence. Ruby 3.2 reached end of life on 2026-04-01 and the backend is on Rails 7.1; the goal is Ruby 3.4.10 and Rails 8.1, in steps that each keep CI green and are independently deployable.

Start with new_framework_defaults_7_2.rb and compare its options with new_framework_defaults_7_1.rb, especially the serialization-related settings and deploy-ordering annotations. Confirm #883 is deployed and soaked, then enable the options incrementally, set config.load_defaults to 7.2, delete the defaults file, and run the full suite; done requires green CI and a separate deployment.