fix: add cast to satisfy ty type checker for list covariance

The ty type checker correctly flags that list[str] is not a subtype of list[str | None] due to list invariance. Added explicit cast.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

django/aiservice/core/log_features/log_features.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, cast
 
 import sentry_sdk
 from asgiref.sync import sync_to_async
@@ -20,7 +20,7 @@ features_api = NinjaAPI(urls_namespace="log_features")
 def _positional_list(items: list[str] | None, index: int | None) -> list[str | None] | None:
     """Place a single item at the correct index, padding with None."""
     if not items or index is None or len(items) != 1:
-        return items
+        return cast("list[str | None] | None", items)
     result: list[str | None] = [None] * (index + 1)
     result[index] = items[0]
     return result