fix(api) : denormalize interface-typed relation collections

Add ContractRelationDenormalizer to resolve IRIs for collections typed
against an interface (Contract), fixing POST/PATCH 400 errors. Cover it
with InterfaceCollectionDenormalizationTest.

src/Shared/Infrastructure/ApiPlatform/Serializer/ContractRelationDenormalizer.php

@@ -0,0 +1,96 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Shared\Infrastructure\ApiPlatform\Serializer;
+
+use ApiPlatform\Metadata\IriConverterInterface;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Component\Serializer\Normalizer\DenormalizerAwareInterface;
+use Symfony\Component\Serializer\Normalizer\DenormalizerAwareTrait;
+use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
+use Throwable;
+
+use function array_key_exists;
+use function is_string;
+
+/**
+ * Modular monolith: cross-module relations are typed with a Shared\Domain\Contract
+ * interface (e.g. UserInterface, TaskTagInterface) instead of the concrete entity,
+ * to keep modules decoupled. Doctrine maps those back to the concrete entity through
+ * resolve_target_entities.
+ *
+ * API Platform denormalizes *single* interface relations fine (the concrete class is
+ * derived from the IRI), but blows up on *collections*: the collection value type stays
+ * the interface, which is not a registered API resource, so no normalizer supports it
+ * and the request fails with NotNormalizableValueException.
+ *
+ * This denormalizer bridges that gap for every contract interface, reusing Doctrine's
+ * resolve_target_entities mapping (no per-entity config):
+ *  - a string value is an IRI -> resolved through the IriConverter
+ *  - an array value is an embedded object -> denormalized into the concrete entity
+ */
+final class ContractRelationDenormalizer implements DenormalizerInterface, DenormalizerAwareInterface
+{
+    use DenormalizerAwareTrait;
+
+    private const CONTRACT_NAMESPACE = 'App\Shared\Domain\Contract\\';
+
+    /** @var array<string, ?class-string> */
+    private array $resolved = [];
+
+    public function __construct(
+        private readonly IriConverterInterface $iriConverter,
+        private readonly EntityManagerInterface $entityManager,
+    ) {}
+
+    public function supportsDenormalization(mixed $data, string $type, ?string $format = null, array $context = []): bool
+    {
+        return null !== $this->concreteClassFor($type);
+    }
+
+    public function denormalize(mixed $data, string $type, ?string $format = null, array $context = []): ?object
+    {
+        $concrete = $this->concreteClassFor($type);
+        if (null === $concrete) {
+            return null;
+        }
+
+        if (is_string($data)) {
+            return $this->iriConverter->getResourceFromIri($data, $context);
+        }
+
+        // Embedded object payload: denormalize into the resolved concrete entity.
+        return $this->denormalizer->denormalize($data, $concrete, $format, $context);
+    }
+
+    public function getSupportedTypes(?string $format): array
+    {
+        // Support depends on the runtime-resolved Doctrine mapping, so it cannot be
+        // statically cached by the serializer.
+        return ['object' => false];
+    }
+
+    /**
+     * @return ?class-string the concrete entity a contract interface resolves to, or null
+     */
+    private function concreteClassFor(string $type): ?string
+    {
+        if (array_key_exists($type, $this->resolved)) {
+            return $this->resolved[$type];
+        }
+
+        if (!str_starts_with($type, self::CONTRACT_NAMESPACE) || !interface_exists($type)) {
+            return $this->resolved[$type] = null;
+        }
+
+        try {
+            $name = $this->entityManager->getClassMetadata($type)->getName();
+        } catch (Throwable) {
+            // Not a Doctrine-mapped (resolve_target_entities) interface.
+            return $this->resolved[$type] = null;
+        }
+
+        return $this->resolved[$type] = ($name !== $type ? $name : null);
+    }
+}