Stopping debusine-signing.service or debusine-worker.service results in a failed systemd state

Expected behavior systemctl stop debusine-signing leaves the service in a successfully exited state. Actual behavior:

2025-09-18 11:09:07,124 Terminated with signal SIGTERM
Traceback (most recent call last):
  File "/usr/bin/debusine-signing", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/__main__.py", line 56, in main
    execute_from_command_line(sys.argv)
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/lib/python3/dist-packages/debusine/django/management/debusine_base_command.py", line 64, in execute
    super().execute(*args, **options)
  File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/signing/management/commands/signing_worker.py", line 57, in handle
    asyncio.run(worker.main())
  File "/usr/lib/python3.11/asyncio/runners.py", line 190, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/asyncio/base_events.py", line 653, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/worker/_worker.py", line 220, in main
    await self.connect()
  File "/usr/lib/python3/dist-packages/debusine/worker/_worker.py", line 386, in connect
    await _do_wait_for_messages()
  File "/usr/lib/python3/dist-packages/tenacity/_asyncio.py", line 88, in async_wrapped
    return await fn(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/tenacity/_asyncio.py", line 47, in __call__
    do = self.iter(retry_state=retry_state)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/tenacity/__init__.py", line 314, in iter
    return fut.result()
           ^^^^^^^^^^^^
  File "/usr/lib/python3.11/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/usr/lib/python3/dist-packages/tenacity/_asyncio.py", line 50, in __call__
    result = await fn(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/worker/_worker.py", line 347, in _do_wait_for_messages
    async with self._aiohttp_client_session.ws_connect(
  File "/usr/lib/python3/dist-packages/aiohttp/client.py", line 1141, in __aenter__
    self._resp = await self._coro
                 ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/aiohttp/client.py", line 779, in _ws_connect
    resp = await self.request(
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/aiohttp/client.py", line 400, in _request
    raise RuntimeError("Session is closed")
RuntimeError: Session is closed
debusine-signing.service: Main process exited, code=exited, status=1/FAILURE
debusine-signing.service: Failed with result 'exit-code'.
Stopped debusine-signing.service - Debusine Signing Worker.