fix: defer Active Record patching via ActiveSupport.on_load

[MattyMc]

Previously, requiring the gem force-loaded active_record/type/json during Bundler.require, before app initializers, so initializers that configure the JSON encoder (e.g., Oj::Rails.set_encoder) had no effect on json/jsonb columns in some Rails versions (e.g. 8.1.2). Now the patches load when ActiveRecord::Base does.

This follows Rails’ recommended pattern for applying configuration to framework classes: use a lazy load hook to avoid autoloading the class before initialization has completed. See the Rails guide note on Avoid Loading Rails Frameworks.

Fixes #60

• I've added tests for this change
• I've added a Changelog entry
• I've updated a documentation (Readme) – N/A

[MattyMc]

Note: the railsmaster CI failure is not caused by this change. It also happens on an untouched master checkout. The latest pglite gem (0.1.1) is missing some connection methods that the latest Rails now calls (close, finished?, status) and its WASM build crashes when Rails sets the session time zone, which edge Rails now does on every new connection. It can be safely ignored.