feat: add GetHandler method and improve push notification API encapsulation

- Add GetHandler() method to PushNotificationProcessorInterface for better encapsulation
- Add GetPushNotificationHandler() convenience method to Client and SentinelClient
- Remove HasHandlers() check from ProcessPendingNotifications to ensure notifications are always consumed
- Use PushNotificationProcessorInterface in internal pool package for proper abstraction
- Maintain GetRegistry() for backward compatibility and testing
- Update pubsub to use GetHandler() instead of GetRegistry() for cleaner code

Benefits:
- Better API encapsulation - no need to expose entire registry
- Cleaner interface - direct access to specific handlers
- Always consume push notifications from reader regardless of handler presence
- Proper abstraction in internal pool package
- Backward compatibility maintained
- Consistent behavior across all processor types

redis.go

@@ -837,6 +837,12 @@ func (c *Client) GetPushNotificationProcessor() PushNotificationProcessorInterfa
 	return c.pushProcessor
 }
 
+// GetPushNotificationHandler returns the handler for a specific push notification name.
+// Returns nil if no handler is registered for the given name.
+func (c *Client) GetPushNotificationHandler(pushNotificationName string) PushNotificationHandler {
+	return c.pushProcessor.GetHandler(pushNotificationName)
+}
+
 type PoolStats pool.Stats
 
 // PoolStats returns connection pool stats.