[lib] Tighten utils type to contain only thick threads

Summary:
It is inconvenient to have the assertions in a lot of places - the issue would become a lot worse after the rest of the stack.

This diff introduces a new risk - an attacker can create a new thread with the same ID as an existing thin thread. Not sure how useful this attack could be. Protecting against it is handled in https://linear.app/comm/issue/ENG-9468/introduce-additional-validation-of-operations.

https://linear.app/comm/issue/ENG-9823/fix-sending-notifs-about-leaving-a-thick-thread

Depends on D13698

Test Plan: Flow

Reviewers: kamil, angelika

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13842