fix: improve error message in close_session method

Changed error message from generic 'Failed to execute command' to more
specific 'Failed to close session' for better debugging clarity when
session closure fails.

Author: bird2426

rock/sdk/sandbox/client.py

@@ -887,7 +887,7 @@ async def close_session(self, request: CloseSessionRequest) -> CloseSessionRespo
 
         logging.debug(f"Close session response: {response}")
         if "Success" != response.get("status"):
-            raise Exception(f"Failed to execute command: {response}")
+            raise Exception(f"Failed to close session: {response}")
         result: dict = response.get("result")  # type: ignore
         return CloseSessionResponse(**result)