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/application/tests/processing/commands/mod.rs

@@ -1,6 +1,8 @@
 mod complete_job;
 mod configure_pipeline;
 mod enqueue_job;
+mod execute_pipeline;
 mod fail_job;
 mod manage_plugin;
+mod process_next_job;
 mod start_job;