I've created a new git repository with JSON Schema definitions for several Fediverse protocols and data formats, including Activity Streams 2.0, #ActivityPub, JSON Resource Description (Webfinger, Nodeinfo), and Mastodon [WIP]. There are automated tests that validate the W3C AS2 test corpus (a fork with bug fixes), examples from the AP spec and Mastodon AP docs, and others.
https://github.com/steve-bate/fediverse-jsonschema