api.email.getThread fetches only the newest 50 messages of a thread
(Thread/get, then Email/get for those ids) instead of all of them with
their bodies. api.email.getThreadCount answers the number of messages
from Thread/get, with its own rate limit.
plugin-host-thread.test.ts now expects getThreadEmails to be called with
{ newest: 50 }, the new option that limits what is fetched.