fix: do not sort prefetched messages by INTERNALDATE

Messages are iterated over in fetch_new_msg_batch()
and largest_uid_fetched variable is updated there
assuming that messages come in the order of increasing UID.
If UIDs are not increasing, it is possible
that largest_uid_fetched will be updated
even though smaller UID is not fetched yet
and the message will be lost.

INTERNALDATE sorting was introduced to
deal with email providers such as Gmail
that keep INTERNALDATE but not the UID
order when moving the messages.
Since we don't move the messages anymore
after commit 04c0e7da16,
there is no need for ordering by INTERNALDATE.

src/imap/session.rs

@@ -16,7 +16,7 @@ use crate::net::session::SessionStream;
 /// - Autocrypt-Setup-Message to check if a message is an autocrypt setup message,
 ///   not necessarily sent by Delta Chat.
 /// - Chat-Is-Post-Message to skip it in background fetch or when it is > `DownloadLimit`.
-const PREFETCH_FLAGS: &str = "(UID INTERNALDATE RFC822.SIZE BODY.PEEK[HEADER.FIELDS (\
+const PREFETCH_FLAGS: &str = "(UID RFC822.SIZE BODY.PEEK[HEADER.FIELDS (\
                               MESSAGE-ID \
                               DATE \
                               X-MICROSOFT-ORIGINAL-MESSAGE-ID \
@@ -124,7 +124,7 @@ impl Session {
     }
 
     /// Prefetch `n_uids` messages starting from `uid_next`. Returns a list of fetch results in the
-    /// order of ascending delivery time to the server (INTERNALDATE).
+    /// order of ascending UIDs.
     #[expect(clippy::arithmetic_side_effects)]
     pub(crate) async fn prefetch(
         &mut self,
@@ -142,10 +142,10 @@ impl Session {
         let mut msgs = BTreeMap::new();
         while let Some(msg) = list.try_next().await? {
             if let Some(msg_uid) = msg.uid {
-                msgs.insert((msg.internal_date(), msg_uid), msg);
+                msgs.insert(msg_uid, msg);
             }
         }
 
-        Ok(msgs.into_iter().map(|((_, uid), msg)| (uid, msg)).collect())
+        Ok(Vec::from_iter(msgs))
     }
 }