Saving Shipment overwrites its associated Inventory Source with 'default'

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### Preconditions (*)

Magento 2.4.2 (tested with a vanilla-ish installation, i.e. disabling all non-core modules)

composer info | grep module-inventory

magento/module-inventory                                          1.2.0     N/A

magento/module-inventory-admin-ui                                 1.2.0     N/A

magento/module-inventory-advanced-checkout                        1.2.0     N/A

magento/module-inventory-api                                      1.2.0     N/A

magento/module-inventory-bundle-import-export                     1.1.0     N/A

magento/module-inventory-bundle-product                           1.2.0     N/A

magento/module-inventory-bundle-product-admin-ui                  1.2.0     N/A

magento/module-inventory-bundle-product-indexer                   1.1.0     N/A

magento/module-inventory-cache                                    1.2.0     N/A

magento/module-inventory-catalog                                  1.2.0     N/A

magento/module-inventory-catalog-admin-ui                         1.2.0     N/A

magento/module-inventory-catalog-api                              1.3.0     N/A

magento/module-inventory-catalog-frontend-ui                      1.0.0     N/A

magento/module-inventory-catalog-search                           1.2.0     N/A

magento/module-inventory-configurable-product                     1.2.0     N/A

magento/module-inventory-configurable-product-admin-ui            1.2.0     N/A

magento/module-inventory-configurable-product-frontend-ui         1.0.0     N/A

magento/module-inventory-configurable-product-indexer             1.2.0     N/A

magento/module-inventory-configuration                            1.2.0     N/A

magento/module-inventory-configuration-api                        1.2.0     N/A

magento/module-inventory-distance-based-source-selection          1.2.0     N/A

magento/module-inventory-distance-based-source-selection-admin-ui 1.2.0     N/A

magento/module-inventory-distance-based-source-selection-api      1.2.0     N/A

magento/module-inventory-elasticsearch                            1.2.0     N/A

magento/module-inventory-export-stock                             1.2.0     N/A

magento/module-inventory-export-stock-api                         1.2.0     N/A

magento/module-inventory-graph-ql                                 1.2.0     N/A

magento/module-inventory-grouped-product                          1.2.0     N/A

magento/module-inventory-grouped-product-admin-ui                 1.2.0     N/A

magento/module-inventory-grouped-product-indexer                  1.2.0     N/A

magento/module-inventory-import-export                            1.2.0     N/A

magento/module-inventory-in-store-pickup                          1.1.0     N/A

magento/module-inventory-in-store-pickup-admin-ui                 1.1.0     N/A

magento/module-inventory-in-store-pickup-api                      1.1.0     N/A

magento/module-inventory-in-store-pickup-frontend                 1.1.0     N/A

magento/module-inventory-in-store-pickup-graph-ql                 1.1.0     N/A

magento/module-inventory-in-store-pickup-multishipping            1.1.0     N/A

magento/module-inventory-in-store-pickup-quote                    1.1.0     N/A

magento/module-inventory-in-store-pickup-quote-graph-ql           1.1.0     N/A

magento/module-inventory-in-store-pickup-sales                    1.1.0     N/A

magento/module-inventory-in-store-pickup-sales-admin-ui           1.1.0     N/A

magento/module-inventory-in-store-pickup-sales-api                1.1.0     N/A

magento/module-inventory-in-store-pickup-shipping                 1.1.0     N/A

magento/module-inventory-in-store-pickup-shipping-admin-ui        1.1.0     N/A

magento/module-inventory-in-store-pickup-shipping-api             1.1.0     N/A

magento/module-inventory-in-store-pickup-webapi-extension         1.1.0     N/A

magento/module-inventory-indexer                                  2.1.0     N/A

magento/module-inventory-low-quantity-notification                1.2.0     N/A

magento/module-inventory-low-quantity-notification-admin-ui       1.2.0     N/A

magento/module-inventory-low-quantity-notification-api            1.2.0     N/A

magento/module-inventory-multi-dimensional-indexer-api            1.2.0     N/A

magento/module-inventory-product-alert                            1.2.0     N/A

magento/module-inventory-requisition-list                         1.2.0     N/A

magento/module-inventory-reservation-cli                          1.2.0     N/A

magento/module-inventory-reservations                             1.2.0     N/A

magento/module-inventory-reservations-api                         1.2.0     N/A

magento/module-inventory-sales                                    1.2.0     N/A

magento/module-inventory-sales-admin-ui                           1.2.0     N/A

magento/module-inventory-sales-api                                1.2.0     N/A

magento/module-inventory-sales-frontend-ui                        1.2.0     N/A

magento/module-inventory-setup-fixture-generator                  1.2.0     N/A

magento/module-inventory-shipping                                 1.2.0     N/A

magento/module-inventory-shipping-admin-ui                        1.2.0     N/A

magento/module-inventory-source-deduction-api                     1.2.0     N/A

magento/module-inventory-source-selection                         1.2.0     N/A

magento/module-inventory-source-selection-api                     1.4.0     N/A

magento/module-inventory-swatches-frontend-ui                     1.0.0     N/A

magento/module-inventory-visual-merchandiser                      1.1.0     N/A

magento/module-inventory-wishlist                                 1.0.0     N/A

### Steps to reproduce (*)

```
// $om = instance of object manager
// wrap in transaction in order to make the test repeatable

/** @var \Magento\Sales\Model\ResourceModel\Order\Shipment\Collection $shipmentCollection */
$shipmentCollection = $om->get(\Magento\Sales\Model\ResourceModel\Order\Shipment\CollectionFactory::class)->create();
$shipmentCollection->setPageSize(2);
$shipmentCollection->setOrder('created_at', \Magento\Framework\Data\Collection::SORT_ORDER_ASC);

$connection = $resourceConnection->getConnection();

/** @var \Magento\Sales\Model\Order\Shipment $shipment */
foreach ($shipmentCollection as $shipment) {
$shipmentSourceSelect = $connection->select();
$shipmentSourceSelect->from($connection->getTableName('inventory_shipment_source'));
$shipmentSourceSelect->where('shipment_id = ?', $shipment->getId());
$shipmentSourceSelect->columns('source_code');

$shipmentSourceData = $connection->fetchRow($shipmentSourceSelect);

$before = $shipmentSourceData['source_code'];

$shipment->setHasDataChanges(true);
$shipment->getResource()->save($shipment);

$shipmentSourceData = $connection->fetchRow(clone $shipmentSourceSelect);

$after = $shipmentSourceData['source_code'];

printf(
"Shipment %d\nBefore: %s\nAfter: %s\n\n",
$shipment->getId(),
$before,
$after,
);
}
```

### Expected result (*)
```
Shipment 42
Before: some_source_code_1
After: some_source_code_1

Shipment 43
Before: some_source_code_2
After: some_source_code_2
```

### Actual result (*)
```
Shipment 42
Before: some_source_code_1
After: default

Shipment 43
Before: some_source_code_2
After: default
```

### Root cause, additional information
`\Magento\InventoryShipping` handles saving of the `sourceCode` Shipment's extension attribute in \Magento\InventoryShipping\Plugin\Sales\ResourceModel\Order\Shipment\SaveSourceForShipmentPlugin::afterSave, and defaults to `default` in case it was not previously hydrated.
The issue is that the only hydration points I found so far for such extension attribute
- \Magento\InventoryShipping\Plugin\Sales\Model\Order\GetListShipmentRepositoryPlugin::afterGetList
- \Magento\InventoryShipping\Plugin\Sales\ResourceModel\Order\Shipment\LoadSourceForShipmentPlugin::afterLoad
- \Magento\InventoryShipping\Plugin\Sales\Shipment\AssignSourceCodeToShipmentPlugin::afterCreate

, do _not_ cover the Shipment being loaded via collection such as in the repro snippet above, which emulates the real word scenario of async shipment email sending (which is what let me discover the bug in the first place):
```
// \Magento\Sales\Model\EmailSenderHandler::sendEmails:132

/** @var \Magento\Sales\Model\AbstractModel $item */
foreach ($entityCollection->getItems() as $item) {
if ($this->emailSender->send($item, true)) {
$this->entityResource->save(
$item->setEmailSent(true)
);
}
}
```

Start with Magento\InventoryShipping\Plugin\Sales\ResourceModel\Order\Shipment\SaveSourceForShipmentPlugin::afterSave and compare it with LoadSourceForShipmentPlugin::afterLoad and GetListShipmentRepositoryPlugin::afterGetList. Reproduce the issue using a shipment collection and inspect inventory_shipment_source before and after saving. Done means collection-loaded shipments retain their existing source_code, with coverage for the reported scenario.

Written by the indexing model from the issue text.