fehler mit fehlendem Context-Sheet gefixed

math-tutor/backend/app/deterministic_services/context_stores/context_store_base.py

@@ -23,10 +23,11 @@ def _latest_filename_pattern(chat_id: str) -> str:
     return os.path.join(_LOG_DIR, f"*_{chat_id}_latest.json")
 
 
-def _resolve_latest_path(chat_id: str) -> str:
+def _resolve_latest_path(chat_id: str) -> str | None:
     matches = glob(_latest_filename_pattern(chat_id))
     if matches:
         return max(matches, key=os.path.getmtime)
+    return None
 
 
 def _utc_now() -> str:
@@ -73,7 +74,7 @@ def load_sheet(chat_id: str) -> dict[str, Any] | None:
             return _CACHE[chat_id]
 
     latest_path = _resolve_latest_path(chat_id)
-    if os.path.exists(latest_path):
+    if latest_path and os.path.exists(latest_path):
         with open(latest_path, "r", encoding="utf-8") as file_handle:
             sheet = json.load(file_handle)
         with _LOCK: