Magento\Tax\Model\Quote\ToOrderConverter uses wrong type for extension attribute

[fooman]

Preconditions (*)

1. Magento 2.4-develop

Steps to reproduce (*)

1. Look at code
   https://github.com/magento/magento2/blob/2.1/app/code/Magento/Tax/Model/Quote/ToOrderConverter.php

   $itemAppliedTaxesModified = [];
   if (!empty($itemAppliedTaxes)) {
       foreach ($itemAppliedTaxes as $key => $itemAppliedTaxItem) {
           if (is_array($itemAppliedTaxItem) && !empty($itemAppliedTaxItem)) {
               foreach ($itemAppliedTaxItem as $itemAppliedTax) {
                   $itemAppliedTaxesModified[$key]['type'] = $itemAppliedTax['item_type'];
                   $itemAppliedTaxesModified[$key]['item_id'] = $itemAppliedTax['item_id'];
                   $itemAppliedTaxesModified[$key]['associated_item_id'] = $itemAppliedTax['associated_item_id'];
                   $itemAppliedTax['extension_attributes']['rates'] = $itemAppliedTax['rates'];
                   unset($itemAppliedTax['rates']);
                   $itemAppliedTaxesModified[$key]['applied_taxes'][] = $itemAppliedTax;
               }
           }
       }
       $extensionAttributes->setItemAppliedTaxes($itemAppliedTaxesModified);
   

vs

https://github.com/magento/magento2/blob/2.1/app/code/Magento/Tax/etc/extension_attributes.xml#L14

<extension_attributes for="Magento\Sales\Api\Data\OrderInterface">
    <attribute code="applied_taxes" type="Magento\Tax\Api\Data\OrderTaxDetailsAppliedTaxInterface[]" />

resulting in Magento\Sales\Api\Data\OrderExtensionInterface (in generated folder)

/**
 * @return \Magento\Tax\Api\Data\OrderTaxDetailsItemInterface[]|null
 */
public function getItemAppliedTaxes();

/**
 * @param \Magento\Tax\Api\Data\OrderTaxDetailsItemInterface[] $itemAppliedTaxes
 * @return $this
 */
public function setItemAppliedTaxes($itemAppliedTaxes);

Expected result (*)

1. ExtensionAttributes should be used with the defined interface: array of Magento\Tax\Api\Data\OrderTaxDetailsItemInterface
2. A developer can trust the stated defined return value

Actual result (*)

1. a standard php array is used

[fooman]

@andimov thanks for looking into this - I'd currently would label this a bug rather than an improvement