[keyserver] Update tID in register response validator

Summary:
Fixes ENG-4411

threadInfos is a dict from threadID to threadInfo, so the domain should be marked as tID.

Test Plan: Try registering and confirm that there is no error. Try logging into the newly created account.

Reviewers: kamil, inka

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8546