fix: use backend in Cursor, not thrift_backend

Signed-off-by: varun-edachali-dbx <varun.edachali@databricks.com>

Author: varun-edachali-dbx

src/databricks/sql/client.py

@@ -1138,7 +1138,7 @@ def close(self) -> None:
         # Close active operation handle if it exists
         if self.active_op_handle:
             try:
-                self.thrift_backend.close_command(self.active_op_handle)
+                self.backend.close_command(self.active_op_handle)
             except RequestError as e:
                 if isinstance(e.args[1], CursorAlreadyClosedError):
                     logger.info("Operation was canceled by a prior request")