Pin json below 3.0 for the test suite

[snaka]

Summary

json 3.0.0 (released 2026-09-07) removed the quirks_mode option, but ActiveSupport 7.2.3.x still passes it to JSON.generate. Once ActiveSupport's JSON extensions are loaded, Hash#to_json raises:

ArgumentError:
  unknown keyword: quirks_mode
# .../json-3.0.2/lib/json/common.rb:382:in `generate'
# .../activesupport-7.2.3.2/lib/active_support/json/encoding.rb:110:in `stringify'

main has not changed since its last green run (2026-08-09, fa42fc0). That run predates json 3.0. Because Gemfile.lock is not committed, a new run on the same commit now resolves json 3.0.2.

Which jobs are affected

• Ruby >= 3.2 with Rails 7.x: affected. ActiveSupport 7.2.4 no longer passes quirks_mode, but it caps connection_pool < 3. On Ruby >= 3.2, Bundler resolves connection_pool 3.x together with ActiveSupport 7.2.3.2 instead.
• Ruby 3.1 with Rails 7.x: not affected. It resolves ActiveSupport 7.2.4.
• Rails 8.x: not affected. ActiveSupport 8.1 does not pass quirks_mode.

The failure depends on spec order. ActiveSupport is only loaded when spec/persistence/cookie_adapter_spec.rb runs require "rails", so specs that serialize JSON fail only when that file runs earlier under the random order. Running cookie_adapter_spec.rb then experiment_storage_spec.rb with --order defined reproduces it consistently on Ruby 3.2 and 3.3 with Rails 7.x.

Change

This PR adds gem "json", "< 3" to the development Gemfile only. The gemspec is unchanged, so gem users are not affected.

Verification

• Full CI matrix, all 18 jobs pass. It runs on my fork because the workflow only triggers on push: https://github.com/snaka/split/actions/runs/36289493495
• Locally on unmodified main, Ruby 3.3 + RAILS_VERSION=7.1 fails with the error above. With this change, it passes (522 examples, 0 failures).

Note (not addressed here)

gem "rails", "~> #{ENV.fetch('RAILS_VERSION', '8.0')}" expands to ~> 7.1, which allows 7.2, so the "Rails 7.1" jobs actually resolve Rails 7.2.

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