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

pubsub.go

@@ -435,18 +435,16 @@ func (c *PubSub) newMessage(reply interface{}) (interface{}, error) {
 			}, nil
 		default:
 			// Try to handle as generic push notification
-			if c.pushProcessor.IsEnabled() {
-				ctx := c.getContext()
-				registry := c.pushProcessor.GetRegistry()
-				if registry != nil {
-					handled := registry.HandleNotification(ctx, reply)
-					if handled {
-						// Return a special message type to indicate it was handled
-						return &PushNotificationMessage{
-							Command: kind,
-							Args:    reply[1:],
-						}, nil
-					}
+			ctx := c.getContext()
+			registry := c.pushProcessor.GetRegistry()
+			if registry != nil {
+				handled := registry.HandleNotification(ctx, reply)
+				if handled {
+					// Return a special message type to indicate it was handled
+					return &PushNotificationMessage{
+						Command: kind,
+						Args:    reply[1:],
+					}, nil
 				}
 			}
 			return nil, fmt.Errorf("redis: unsupported pubsub message: %q", kind)