[lib] Refactor fetchDerivedMessages to call isInvalidSidebarSource or isInvalidPinSource

Summary:
Now that we have both isInvalidSidebarSource and isInvalidPinSource, it makes sense to make the checks in fetchDerivedMessages more specific. Rather than simply filtering out message types generically, we'll handle it more specifically with 1) checking the message type and 2) calling either isInvalidSidebarSource or isInvalidPinSource accordingly.

Once again, there's a FlowFixMe for the same reason as in D9537 -- Flow doesn't refine the types if I don't explicitly invariant on all of the unexpected message types, and that list can either get long or just generally defeats the purpose of moving the logic into message specs to have one single source of truth.

Resolves https://linear.app/comm/issue/ENG-5218/refactor-fetchderivedmessages-to-call-isinvalidsidebarsource-or

Depends on D9537

Test Plan: yarn workspace lib test and repeated the same test plan as D9537 to ensure usage of the app doesn't trigger unexpected invariants

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9538