This update is more convenient because it doesn't override the whole thread, which fixes a bug where the unread status operation sometimes swallows other operations. For the thick threads, this update should also bump a timestamp.

https://linear.app/comm/issue/ENG-9343/updating-thread-unread-status-sometimes-swallows-queued-operations

1. Send a message as one user and check if a thread of the other used got marked as unread
2. Send a message as a user and check if a thread doesn't get marked as unread on another device of the same user
3. Check if marking as unread results in a thread being marked as unread on another device of the same user