revert credit validator

Author: giancarloromeo

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

@@ -13,7 +13,7 @@ class WalletTotalCredits(BaseModel):
     wallet_id: WalletID
     available_osparc_credits: Decimal
 
-    @field_validator("available_osparc_credits", mode="after")
+    @field_validator("available_osparc_credits")
     @classmethod
     def ensure_rounded(cls, v):
         return round(v, 2)