style(gateway): drop a constant getattr and explicit None returns

Current main enables ruff's B009 and RET501, which flag a constant `getattr` in the completion delivery code and two explicit `return None` statements in the requeue helper once this branch is merged. No behaviour change.

gateway/run_notifications.py

@@ -1294,7 +1294,7 @@ class GatewayNotificationsMixin(GatewayNotificationOwnershipMixin):
         entry = None
         if session_key:
             try:
-                session_store = cast(SessionStore, getattr(self, "session_store"))
+                session_store: SessionStore = cast(Any, self).session_store
                 await asyncio.to_thread(session_store._ensure_loaded)
                 entry = session_store._entries.get(session_key)
             except Exception:

gateway/run_notifications_ownership.py

@@ -211,7 +211,7 @@ class GatewayNotificationOwnershipMixin:
         """Offer admitted completions again when the turn could not confirm their owner (``retry``),
         so the next delivery re-runs the pre-flight; boundary verdicts stay dropped."""
         if verdict != "retry" or not completion_ids:
-            return None
+            return
         from tools.async_delegation import requeue_admitted_completion
         from tools.process_registry import process_registry
         runner = cast(Any, self)
@@ -229,4 +229,3 @@ class GatewayNotificationOwnershipMixin:
                 "Async-delegation completion %s: owner not confirmed after admission; %s.", delegation_id,
                 "offered again" if requeued else "not offered again (result remains queryable)",
             )
-        return None