refactor: remove unnecessary enabled field and IsEnabled/SetEnabled methods

- Remove enabled field from PushNotificationProcessor struct
- Remove IsEnabled() and SetEnabled() methods from processor interface
- Remove enabled parameter from NewPushNotificationProcessor()
- Update all interfaces in pool package to remove IsEnabled requirement
- Simplify processor logic - if processor exists, it works
- VoidPushNotificationProcessor handles disabled case by discarding notifications
- Update all tests to use simplified interface without enable/disable logic

Benefits:
- Simpler, cleaner interface with less complexity
- No unnecessary state management for enabled/disabled
- VoidPushNotificationProcessor pattern handles disabled case elegantly
- Reduced cognitive overhead - processors just work when set
- Eliminates redundant enabled checks throughout codebase
- More predictable behavior - set processor = it works

internal/pool/conn.go

@@ -28,7 +28,6 @@ type Conn struct {
 
 	// Push notification processor for handling push notifications on this connection
 	PushNotificationProcessor interface {
-		IsEnabled() bool
 		ProcessPendingNotifications(ctx context.Context, rd *proto.Reader) error
 	}
 }