imap: read_thread rejects a bare Message-ID the contract says it accepts #1924

What's broken

imap_source.read_thread rejects a bare Message-ID ([email protected]),
although the mail-source contract says that form is accepted on input.

Which layer

tools/mail-source/imap/ — the read_thread operation of
contract:mail-source.

How to reproduce

$ imap-source-read [email protected]
reading thread [email protected] in INBOX on imap.example.org
thread [email protected] not found in INBOX

The same id with its brackets resolves:

$ imap-source-read '<[email protected]>'

maildir's read_thread accepts both spellings — _thread_key brackets a bare
id before the lookup — so the two backends disagree on their own input.

Expected vs actual

Expected. tools/mail-source/contract.md § Thread identity (added in
#1811):

An id typed without brackets is accepted on input and normalised to the
bracketed form; it is never emitted.

Actual. read_thread compares the argument verbatim against
position.message_id, which is always bracketed
(imap_source.threads.normalise_message_id), so a bare id matches nothing.

Impact. Automated callers are unaffected: list_recent_threads only emits
the bracketed form, so a tracker field written from it round-trips. This bites a
human typing the id, and a tracker field edited by hand.

Surface area (optional)

imap_source.read_thread. imap_source.thread_url takes the same argument and
should be checked in the same pass.

Suggested fix: normalise at the entry point with
imap_source.threads.normalise_message_id, and add a test next to
test_read_thread_declares_missing_root.

Found while implementing #1919 (PR #1922); left out of that PR to keep it to the
issue's scope.