fix(serialization): update from_plain calls to use new signature

Update all from_plain usages to pass factory ref as second parameter:
- Message class registration in message.py
- Custom external example
- Test for custom registration

Signed-off-by: Sakeeb91 <rahman.sakeeb@gmail.com>

Author: Sakeeb91

python/beeai_framework/backend/message.py

@@ -391,7 +391,7 @@ def _to_plain(message: Message[Any]) -> dict[str, Any]:
                 "content": [fragment.model_dump() for fragment in message.content],
             }
 
-        def _from_plain(payload: Mapping[str, Any]) -> Message[Any]:
+        def _from_plain(payload: Mapping[str, Any], ref: type[Any]) -> Any:
             content_payload = payload.get("content", []) or []
             content_models: list[Any] = []
 
@@ -408,13 +408,13 @@ def _from_plain(payload: Mapping[str, Any]) -> Message[Any]:
                 model_cls = allowed_content.get(content_type)
                 if model_cls is None:
                     raise SerializerError(
-                        f"Unsupported message content '{content_type}' for {message_cls.__name__}.",
+                        f"Unsupported message content '{content_type}' for {ref.__name__}.",
                     )
                 content_models.append(model_cls.model_validate(dict(item)))
 
             meta = payload.get("meta") or {}
             meta_copy = dict(meta)
-            return message_cls(content_models, meta=meta_copy, id=payload.get("id"))
+            return ref(content_models, meta=meta_copy, id=payload.get("id"))
 
         Serializer.register(
             message_cls,

python/examples/serialization/custom_external.py

@@ -19,7 +19,7 @@ class ApiToken:
         "value": token.value,
         "expires_at": token.expires_at,
     },
-    from_plain=lambda payload: ApiToken(
+    from_plain=lambda payload, ref: ref(
         value=payload["value"],
         expires_at=payload["expires_at"],
     ),

python/tests/serialization/test_serializer.py

@@ -48,7 +48,7 @@ class ApiToken:
             "value": token.value,
             "expires_at": token.expires_at,
         },
-        from_plain=lambda payload: ApiToken(
+        from_plain=lambda payload, ref: ref(
             value=payload["value"],
             expires_at=payload["expires_at"],
         ),