refactor: code smell fixes — tests, events, naming

- Tests for ExecutePipelineHandler (happy path, fallback, disabled skip, failure retry, not found)
- Tests for ProcessNextJobHandler (empty queue, process, drain multiple)
- DerivativeGenerated domain event + event-payload mapping + event_store aggregate
- Renamed event-payload → adapters-event-payload, event-transport → adapters-event-transport

crates/adapters/nats/Cargo.toml

@@ -5,7 +5,7 @@ edition = "2024"
 
 [dependencies]
 domain          = { workspace = true }
-event-transport = { workspace = true }
+adapters-event-transport = { workspace = true }
 async-nats      = { workspace = true }
 async-trait     = { workspace = true }
 futures         = { workspace = true }