refactor(rivetkit-core): rename StopReason to ShutdownKind and kill enum fallthroughs#4765

NathanFlurry wants to merge 1 commit intosleep-cleanup/11-info-log-envoy-commands-eventsrivet-dev/rivet:sleep-cleanup/11-info-log-envoy-commands-eventsfrom

sleep-cleanup/17-stop-reason-and-fallthrough-fixrivet-dev/rivet:sleep-cleanup/17-stop-reason-and-fallthrough-fixCopy head branch name to clipboard