This diff does two things:

1. Updates useGetMessageInfoForPreview so that it falls back to the old behavior if it fails to find a message to preview that passes showInMessagePreview. This allows us to avoid showing "No messages". This is "Potential solution 2" described on Linear here.
2. Updates useGetMessageInfoForPreview to also return shouldFetchOlderMessages, a flag that indicates to the caller if the fallback described above occurred. This will allow us to implement "Potential solution 3" from the Linear comment linked above.

Tested in combination with the rest of the stack:

1. Make sure membership operations (user joining / leaving) don't appear in MessagePreview
2. Make sure reactions to the viewer's messages still appear in MessagePreview
3. Make sure reactions to other user's messages don't appear in MessagePreview
4. Test fresh login to thread with only one message in the last 14 days, which is a reaction to a non-viewer message. Make sure the reaction initially appears in MessagePreview, but then is replaced after more messages are fetched by the client