Make typing more strict in IAS modification workflow

gso/workflows/l3_core_service/ias/modify_ias.py

 """Modification workflow for a IAS subscription."""
 
 from orchestrator import begin, conditional, done, workflow
-from orchestrator.domain import SubscriptionModel
 from orchestrator.targets import Target
 from orchestrator.workflow import StepList
 from orchestrator.workflows.steps import resync, store_process_subscription, unsync
@@ -10,6 +9,7 @@ from pydantic_forms.core import FormPage
 from pydantic_forms.types import FormGenerator, UUIDstr
 
 from gso.products.product_blocks.ias import IASFlavor
+from gso.products.product_types.ias import IAS
 from gso.workflows.l3_core_service.base_modify_l3_core_service import (
     Operation,
     create_new_sbp,
@@ -25,12 +25,12 @@ def modify_ias_input_form_generator(subscription_id: UUIDstr) -> FormGenerator:
     initial_generator = initial_input_form_generator(subscription_id)
     initial_user_input = yield from initial_generator
 
-    subscription = SubscriptionModel.from_subscription(subscription_id)
+    subscription = IAS.from_subscription(subscription_id)
 
     # Additional IAS step
     class IASExtraForm(FormPage):
         # TODO: remove type hint workaround
-        ias_flavor: IASFlavor | str = subscription.ias.ias_flavor  # type: ignore[attr-defined]
+        ias_flavor: IASFlavor | str = subscription.ias.ias_flavor
 
     ias_extra = yield IASExtraForm
     return initial_user_input | ias_extra.model_dump()