fix(api-models): usage unit to optional (#1073)

Author: hassiebp

langfuse/api/resources/commons/types/model.py

@@ -31,7 +31,7 @@ class Model(pydantic_v1.BaseModel):
     Apply only to generations which are newer than this ISO date.
     """
 
-    unit: ModelUsageUnit = pydantic_v1.Field()
+    unit: typing.Optional[ModelUsageUnit] = pydantic_v1.Field(default=None)
     """
     Unit used by this model.
     """