@-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`.
Linear task: https://linear.app/comm/issue/ENG-6640/mentioning-a-user-in-a-pending-genesis-sidebar-causes-the-app-to-crash