imap: key a reply that carries only In-Reply-To on its parent, as the mail-source contract requires #1919

The mail-source contract's Thread identity section (added in #1811) keys a thread on the first References id, else the first In-Reply-To id, else the message's own Message-ID, even when that root is not in the archive. The maildir backend does all three. The imap backend skips the middle step: imap_source.searching.thread_root_of returns the first References id or the message's own id, so a reply carrying only In-Reply-To is its own thread on IMAP and joins its parent on maildir. An id recorded by a -sync run against one backend then fails to match on the other, and a duplicate tracker gets proposed. The contract and the maildir README currently note this gap.

Two changes close it:

• thread_root_of (tools/mail-source/imap/src/imap_source/searching.py) falls back to the first bracketed In-Reply-To token before the message's own id. Only bracketed tokens count, so a comment beside the id (In-Reply-To: (was: report) <r@h>) is not taken as the root.
• read_thread (tools/mail-source/imap/src/imap_source/read_thread.py) currently refuses a thread whose root message is not in the fetched window (thread root … not found). With the fallback, a thread can be keyed on a root the window does not hold, so it should return the reachable messages instead, the way maildir does, and only fail when nothing references the id.

Done when: both backends give the same thread_id for a reply carrying only In-Reply-To and for replies to a root missing from the archive, with a test for each in the IMAP suite, and the "does not take the In-Reply-To step yet" notes are removed from tools/mail-source/contract.md, tools/maildir/README.md and the read_archive.py docstring.