gh-153494: Encode imaplib search criteria to the declared CHARSET (GH-153495)

gh-153494: Encode imaplib search criteria to the declared CHARSET (GH-153495)

IMAP4.search(), sort(), thread() and the uid SORT/THREAD variants now encode
str search criteria to the declared charset, so international search text can
be passed as ordinary str.  A criterion passed as bytes is sent unchanged, for
use with a charset that Python has no codec for.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>