fix: annotate already_added_keys to satisfy mypy

The parameter-level annotation on _check_keys_and_notif_id alone
wasn't enough -- mypy needs the type at the empty list's declaration
site inside filter_notification_alerts. Also reformatted the helper's
now-longer signature to satisfy ruff.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

orchestrator/activities/slot_manager.py

@@ -420,7 +420,7 @@ class SlotManager(SientiaMonitoring):
             receiver_groups[group_name] = {**receiver_group, 'notifications': []}
             receiver_groups[group_name]['notifications'] = []
 
-            already_added_keys = []
+            already_added_keys: list[str] = []
 
             ignore_list = receiver_group.get('ignore', [])
             levels_list = receiver_group.get('levels')
@@ -486,7 +486,9 @@ class SlotManager(SientiaMonitoring):
         return level_check and model_id_check
 
     @staticmethod
-    def _check_keys_and_notif_id(notification_id, ignore_list, key, already_added_keys) -> bool:
+    def _check_keys_and_notif_id(
+        notification_id, ignore_list, key, already_added_keys: list[str]
+    ) -> bool:
         """
         Check if the key has already been added to the list of already added keys.