Add no split rule for workflow validation

[MAILPOET-4629]

mailpoet/lib/Automation/Engine/Validation/WorkflowRules/NoSplitRule.php

@@ -0,0 +1,23 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Validation\WorkflowRules;
+
+use MailPoet\Automation\Engine\Data\Workflow;
+use MailPoet\Automation\Engine\Exceptions;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNode;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNodeVisitor;
+
+class NoSplitRule implements WorkflowNodeVisitor {
+  public function initialize(Workflow $workflow): void {
+  }
+
+  public function visitNode(Workflow $workflow, WorkflowNode $node): void {
+    $step = $node->getStep();
+    if (count($step->getNextSteps()) > 1) {
+      throw Exceptions::workflowStructureNotValid(__('Path split found in workflow graph', 'mailpoet'));
+    }
+  }
+
+  public function complete(Workflow $workflow): void {
+  }
+}