feat: allow JsonSerializable objects in contract (#377)

As brought up in #371 it could be a possible extension point to the
contract to reintroduce the support for `JsonSerializable` objects when
there are no normalizers for an object.

So it would be a possible alternative for custom `MessageBagInterface`,
`MessageInterface`, `ContentInterface`, etc. by allowing them to
implement the `JsonSerializable` interface.

Author: DZunke

src/platform/src/Contract.php

@@ -24,6 +24,7 @@
 use Symfony\AI\Platform\Contract\Normalizer\ToolNormalizer;
 use Symfony\AI\Platform\Tool\Tool;
 use Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer;
+use Symfony\Component\Serializer\Normalizer\JsonSerializableNormalizer;
 use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
 use Symfony\Component\Serializer\Serializer;
 
@@ -60,6 +61,9 @@ public static function create(NormalizerInterface ...$normalizer): self
         // Response
         $normalizer[] = new ToolCallNormalizer();
 
+        // JsonSerializable objects as extension point to library interfaces
+        $normalizer[] = new JsonSerializableNormalizer();
+
         return new self(
             new Serializer($normalizer),
         );

src/platform/tests/ContractTest.php

@@ -37,9 +37,12 @@
 use Symfony\AI\Platform\Message\Content\ImageUrl;
 use Symfony\AI\Platform\Message\Message;
 use Symfony\AI\Platform\Message\MessageBag;
+use Symfony\AI\Platform\Message\MessageInterface;
+use Symfony\AI\Platform\Message\Role;
 use Symfony\AI\Platform\Message\SystemMessage;
 use Symfony\AI\Platform\Message\UserMessage;
 use Symfony\AI\Platform\Model;
+use Symfony\Component\Uid\Uuid;
 
 #[Large]
 #[CoversClass(Contract::class)]
@@ -198,6 +201,37 @@ public static function providePayloadTestCases(): iterable
                 'model' => 'gpt-4o',
             ],
         ];
+
+        $customSerializableMessage = new class implements MessageInterface, \JsonSerializable {
+            public function getRole(): Role
+            {
+                return Role::User;
+            }
+
+            public function getId(): Uuid
+            {
+                return Uuid::v7();
+            }
+
+            public function jsonSerialize(): array
+            {
+                return [
+                    'role' => 'user',
+                    'content' => 'This is a custom serializable message.',
+                ];
+            }
+        };
+
+        yield 'MessageBag with custom message from GPT' => [
+            'model' => new GPT(),
+            'input' => new MessageBag($customSerializableMessage),
+            'expected' => [
+                'messages' => [
+                    ['role' => 'user', 'content' => 'This is a custom serializable message.'],
+                ],
+                'model' => 'gpt-4o',
+            ],
+        ];
     }
 
     #[Test]