Solution for [ENG-4553](https://linear.app/comm/issue/ENG-4553/encode-message-chat-mentions).
This diff adds support for chats in `typeaheadTooltipSuggestionTextExtractor`. Chat entry is encoded in the following convention:
```
@[[chat_id:default_text]]
```
When user has access to the chat, mentioned thread name is used, otherwise `default_text` is rendered.
Depends on D8908.