MC-1439: Define API & SPI and mark accordingly

Author: irenelagno

app/code/Magento/PageBuilder/Setup/DataConverter/ChildrenRenderer/Concatenation.php

@@ -17,7 +17,7 @@ class Concatenation implements ChildrenRendererInterface
     /**
      * {@inheritdoc}
      */
-    public function render(array $children, $renderChildCallback) : array
+    public function render(array $children, $renderChildCallback) : string
     {
         $childHtml = '';
         foreach ($children as $childIndex => $childItem) {

app/code/Magento/PageBuilder/Setup/DataConverter/ChildrenRenderer/Row.php

@@ -18,7 +18,7 @@ class Row implements ChildrenRendererInterface
     /**
      * {@inheritdoc}
      */
-    public function render(array $children, $renderChildCallback) : array
+    public function render(array $children, $renderChildCallback) : string
     {
         $childHtml = '';
         $columns = [];

app/code/Magento/PageBuilder/Setup/DataConverter/TreeConverter.php

@@ -197,11 +197,11 @@ function () use ($itemData) {
      * @throws UnableMigrateWithOutParentException
      */
     private function handleRenderException(
-        \Exception $exception, 
+        \Exception $exception,
         RendererInterface $defaultRenderer,
         array $itemData,
         array $itemAdditionalData
-    ) : string {
+    ): string {
         if ($this->isUnseparatableContentType($itemData)) {
             throw new UnableMigrateWithOutParentException(
                 __('Content type can not be migrated with out parent.'),