Fixes an error users were occasionally getting in the reviewer when not logged in.
https://ankihub.sentry.io/issues/6253435393/events/f027550d1cb344b9ad9a5cb6b6c3aa9c/?project=6546414

Also makes some other improvements to the handling of 401 exceptions in the reviewer.

Related issues

https://ankihub.sentry.io/issues/6253435393/events/f027550d1cb344b9ad9a5cb6b6c3aa9c/?project=6546414
https://community.ankihub.net/t/got-an-error-message/397521

Proposed changes

The error was caused by our exception handler raising an exception while handling an 401 AnkiHubHTTPError.
The exception handler raised an exception because it was calling config.save_token(), which in turn called the callbacks in config.token_change_hook.
One of the callbacks (update_feature_flags_in_background) called aqt.mw.taskman.run_in_background.
aqt.mw.taskman.run_in_background runs any pending closures from tasks that just finished.
One of the pending closures re-raised an exception which was raised in the associated background task.

Summarizing, the issue is that calling aqt.mw.taskman.run_in_background() can raise an exception if there were was an exception in a (possibly unrelated) background task that was just run. (When this happens, the new background task is not run.)

This is something that should probably be fixed in Anki.

For now, the fix that this PR applies is to use aqt.mw.taskman.run_on_main() to call functions which call aqt.mw.taskman.run_in_background() in some relevant places. Using aqt.mw.taskman.run_on_main(closure) prevents exceptions raised by the closure from getting backpropagated to the caller.
So, the code which calls aqt.mw.taskman.run_in_background, won't be interrupted the exception. (The background task which it tried to start won't be run though.)

We used this technique before in this decorator:

• Run token_change_hook callbacks using run_on_main
  • With this change, the exception handler won't get exceptions from token_change_hook callbacks when calling config.save_token()
• Run other background tasks in errors.py using run_on_main
• Run feature flag update callbacks using run_on_main

Other fixes/improvements:

• Only call token_change_hook callbacks if token actually changes
• Only add _check_premium_and_notify_buttons_once callback if not present
  • Makes sure that the callback gets added only once. Not sure if multiple callbacks were added previously
• Update reviewer buttons when user signs out
  • When you sign out in the reviewer, the chatbot icon will change to the sleeping robot now
• Fix client tests
  • Unrelated to the rest of the PR
  • Updates the tests to adapt to the web app changes made in https://github.com/AnkiHubSoftware/ankihub/pull/2577

Notes

Logs which were helpful while debugging the error:
https://app.datadoghq.com/logs?query=host%3Ashannons-air.dhcp.swmed.org%20service%3Aankihub_addon&agg_m=count&agg_m_source=base&agg_t=count&cols=host%2Cservice&context_event=AZUBBKq2AABE-P7_kAS1CQAK&fromUser=true&messageDisplay=inline&refresh_mode=sliding&storage=hot&stream_sort=time%2Casc&viz=&from_ts=1739478517555&to_ts=1739480376381&live=false