Add checking when adding a stage class string

src/Pipeline/OptionsProcessor/OptionsStageBuilder.php

@@ -22,7 +22,13 @@ class OptionsStageBuilder
     public function add(string|OptionsStageInterface $stageClass): self
     {
         if (is_string($stageClass)) {
+            if (new \ReflectionClass($stageClass)->implementsInterface(OptionsStageInterface::class)) {
                 $this->stages[] = $this->bag->get($stageClass);
+            } else {
+                throw new \InvalidArgumentException(
+                    sprintf('Stage must be an instance of OptionsStageInterface, %s given', get_debug_type($stage))
+                );
+            }
         } else {
             $this->stages[] = $stageClass;
         }