fix(custom-fields) : match by orderIndex to prevent value loss on rename

When a ModelType's custom field was renamed without sending the field ID,
the service would create a new CustomField instead of reusing the existing
one, orphaning all CustomFieldValues. Now matches by orderIndex as fallback
before name, preserving the link to existing values.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/AbstractApiTestCase.php

@@ -382,6 +382,8 @@ abstract class AbstractApiTestCase extends ApiTestCase
         string $value = 'test value',
         ?Machine $machine = null,
         ?Composant $composant = null,
+        ?Piece $piece = null,
+        ?Product $product = null,
     ): CustomFieldValue {
         $cfv = new CustomFieldValue();
         $cfv->setValue($value);
@@ -392,6 +394,12 @@ abstract class AbstractApiTestCase extends ApiTestCase
         if (null !== $composant) {
             $cfv->setComposant($composant);
         }
+        if (null !== $piece) {
+            $cfv->setPiece($piece);
+        }
+        if (null !== $product) {
+            $cfv->setProduct($product);
+        }
 
         $em = $this->getEntityManager();
         $em->persist($cfv);