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>

store/eventstored_test.go

@@ -24,11 +24,11 @@ func NewMockEventBroadcaster() *MockEventBroadcaster {
 }
 
 func (m *MockEventBroadcaster) Subscribe(namespacePattern string) <-chan *aether.Event {
-	return make(chan *aether.Event)
+	return nil
 }
 
 func (m *MockEventBroadcaster) SubscribeWithFilter(namespacePattern string, filter *aether.SubscriptionFilter) <-chan *aether.Event {
-	return make(chan *aether.Event)
+	return nil
 }
 
 func (m *MockEventBroadcaster) Unsubscribe(namespacePattern string, ch <-chan *aether.Event) {}