When a user had the chat option "Show activity indicator in header" set to "all new messages", and they would get a reply to a thread they're part of, the chat icon in the header would not show the unread bubble indicator.
In order to fix this, the `ChatHeaderIconUnreadIndicator` component will now `showUnreadIndicator` whenever there is either one unread public channel or there are unread threads.
I only added a system spec for this very specific path because I don't want to slow down the whole suite to test for all the various combination of the `chat_header_indicator_preference` values.
Internal ref - t/128874
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