[keyserver][lib] change thread info validators -> thin raw thread info validators

Summary: all four of the updated validators here are only ever used to validate thin threads. therefore, they should be thin thread validators

Test Plan:
https://linear.app/comm/issue/ENG-9937/narrow-raw-thread-info-validators this linear task enumerates all callsites for these validators. i checked that they're only ever used on the keyserver or to validate keyserver responses. it looks like we don't use these validators for E2EE DMs.

i also did some manual testing by sending messages through my keyserver and sending DMs to other test accounts

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13949