fix(imap): always advance expected UIDNEXT to avoid skipping IDLE in a loop

Ensure the client does not busy loop
skipping IDLE if UIDNEXT of the mailbox is higher than
the last seen UID plus 1, e.g. if the message
with UID=UIDNEXT-1 was deleted before we fetched it.

We do not fallback to UIDNEXT=1 anymore
if the STATUS command cannot determine UIDNEXT.
There are no known IMAP servers with broken STATUS so far.
This allows to guarantee that select_with_uidvalidity()
sets UIDNEXT for the mailbox and use it in fetch_new_messages()
to ensure that UIDNEXT always advances even
when there are no messages to fetch.

src/imap/idle.rs

@@ -53,7 +53,7 @@ impl Session {
             if uid_next > expected_uid_next {
                 info!(
                     context,
-                    "Skipping IDLE because UIDNEXT indicates there are new messages."
+                    "Skipping IDLE on {folder:?} because UIDNEXT {uid_next}>{expected_uid_next} indicates there are new messages."
                 );
                 return Ok((self, info));
             }