@-mentioning in a pending GENSIS sidebar would crash the app. This was because in `useMentionTypeaheadChatSuggestions` we were attempting to get the chat mention search index from the `communityThreadIDForGenesisThreads` with a pending thread info id. The reason this is an issue is that we build the `communityThreadIDForGenesisThreads` is built with `resolvedThreadInfos`. This meant that pending sidebar thread ids (which look like this `pending/sidebar/256|145027`) weren't included in the `communityThreadIDForGenesisThreads` object, and was returning an `undefined` `chatSearchIndex`.
The solution I've come up with is to add a check if the thread is pending and then if it is, use the resolved containing threadID of the pending sidebar since that will have the same chat mention candidates as the sidebar thread if it was resolved.
Linear task: https://linear.app/comm/issue/ENG-6640/mentioning-a-user-in-a-pending-genesis-sidebar-causes-the-app-to-crash