add serializer

Author: giancarloromeo

packages/models-library/src/models_library/api_schemas_resource_usage_tracker/credit_transactions.py

@@ -1,7 +1,8 @@
 from datetime import datetime
 from decimal import Decimal
+from typing import Annotated
 
-from pydantic import BaseModel, ConfigDict, field_validator
+from pydantic import BaseModel, BeforeValidator, PlainSerializer
 
 from ..products import ProductName
 from ..resource_tracker import CreditTransactionId
@@ -11,15 +12,11 @@
 
 class WalletTotalCredits(BaseModel):
     wallet_id: WalletID
-    available_osparc_credits: Decimal
-
-    model_config = ConfigDict(json_encoders={Decimal: float})
-
-    @field_validator("available_osparc_credits", mode="before")
-    @classmethod
-    def ensure_rounded(cls, v):
-        return round(v, 2)
-
+    available_osparc_credits: Annotated[
+        Decimal, 
+        BeforeValidator(lambda x: round(x, 2)),
+        PlainSerializer(float, return_type=float, when_used='json')
+    ]
 
 class CreditTransactionCreateBody(BaseModel):
     product_name: ProductName