docs: async def wait_done(self) -> int:
          """Coroutine to wait for subprocess run completion.
  
          Returns:
              The exit code of the subprocess.
  
          """
          await self._done_running_evt.wait()
          if self._exit_code is None:
              raise SublemonLifetimeError(
                  'Subprocess exited abnormally with `None` exit code')
          return self._exit_code