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/event-payload/Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "event-payload"
+name = "adapters-event-payload"
 version = "0.1.0"
 edition = "2024"