fix: skip IDLE if we got unsolicited FETCH (#6130)

This may indicate that there was a new \Seen flag
that we don't want to skip.

Also don't drain unsolicited responses while scanning folders. Now we
only drain unsolicited responses right before IDLE and always redo the
whole fetch cycle if there have been some. Some message in the scanned
folder may not be fetched that would be previously fetched otherwise,
but it will be picked up on the next folder scan.

src/imap/idle.rs

@@ -32,7 +32,7 @@ impl Session {
 
         self.select_with_uidvalidity(context, folder).await?;
 
-        if self.server_sent_unsolicited_exists(context)? {
+        if self.drain_unsolicited_responses(context)? {
             self.new_mail = true;
         }