Ceara/aitt fix new thread behavior

[cearachew]

This is a fix for a bug @MollyPeredo noted where you couldn't create a new thread using the "New thread" button if you are still in a thread started with the new thread button.

This is because we previously set the key for the chat component as 0 for a new thread and the thread id for old threads. We set the key when switching threads to re-set the State of the chat component to clear the message history and reset it from params. On a new thread with key == 0, we don't reset the key to the newly created threadID because we want to keep the messagehistory and view as-is. The key remained until switching threads from the sidebar, which cause problems when trying to create a new thread from a previous new thread, because both keys are 0.

This change just alternates 0 and -1 as keys for new threads. -1 is used so it won't accidentally clash with a real thread ID.

Video:

Screen.Recording.2025-08-20.at.7.13.33.PM.mov

Links

slack thread about bug: https://codedotorg.slack.com/archives/C050HE4QDHR/p1755200424919449?thread_ts=1755196263.126769&cid=C050HE4QDHR
Included in jira ticket: https://codedotorg.atlassian.net/browse/AITT-1115

Testing story

Previous non-thread tests are passing

• Tests provide adequate coverage
• Privacy impacts have been documented
• Security impacts have been documented
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Follow-up work items (including potential tech debt) are tracked and linked