From rMW407f679bd5c4: doc: Make extension.schema.v2.json a valid JSON schema., it seems the file wasn't valid JSON to the schema...
We should (ideally) have a test in MW core to do this. I'm not sure how practical this may be due to the need to resolve external references...
We already include justinrainbow/json-schema, so should be able to make use of this for the purpose...
Will probably need to be rewritten eventually for T293710: Remove opis/json-schema from MW vendor now that it's no longer used, everyone's on justinrainbow/json-schema, but MW core doesn't include it yet, and it's down the road...