fix: address review feedback

- Removed duplicate blank line in event.go
- Use original event timestamp instead of time.Now() for EventStored
- Fixed MockEventBroadcaster.Subscribe to return nil instead of closed channel
- Added integration tests for EventStored with JetStreamEventStore

Co-Authored-By: Claude Code <noreply@anthropic.com>

event.go

@@ -166,7 +166,6 @@ func (e *Event) WithMetadataFrom(source *Event) {
 	}
 }
 
-
 // EventStored is an internal infrastructure event published after an event is successfully persisted.
 // It allows observability and trigger downstream workflows without coupling to application events.
 // EventStored is not published to external systems (Phase 2) - only to local EventBus subscribers.