added type hint to _safe_get_session_details

Author: ShaharKatz

src/google/adk/evaluation/local_eval_service.py

@@ -485,7 +485,7 @@ def _resolve_user_id(self, eval_case: EvalCase) -> str:
 
   async def _safe_get_session_details(
       self, *, app_name: str, user_id: str, session_id: str
-  ):
+  ) -> Optional['Session']:
     try:
       return await self._session_service.get_session(
           app_name=app_name, user_id=user_id, session_id=session_id