[lib] Add invariant to SidebarSource notificationTexts

Summary:
While we do indicate SidebarSource generates notifs in generatesNotifs, it should always be collapsed, and should never appear as the most recent message for a given collapse key (since CreateSidebar is created at the same time and always is considered "newer").

To make sure that notificationTexts is never called for SidebarSource, I'm adding this invariant here.

Depends on D6869

Test Plan: Made sure this invariant never gets triggered in a variety of sidebar creation scenarios

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6870