## What changed?
Support Nexus async completion in CHASM.
PS: completion-before-start will be handled in a follow-up PR.
## How did you test it?
- [ ] built
- [ ] run locally and tested manually
- [ ] covered by existing tests
- [x] added new unit test(s)
- [x] added new functional test(s)
## Potential risks
Is behind feature flag.