fix(checkpoint): drop unroundtrippable callbacks and adapter state

- callable_to_string returns None for lambdas/closures instead of an
  unresolvable dotted path; Crew filters Nones out of restored callback
  lists.
- EventNode.event serializer honors info.mode so mode='json' calls cascade
  properly into nested event payloads.
- RagTool.adapter serializes to None (post-validator rebuilds from
  config); concrete adapters hold runtime state that can't be round-tripped.