refactor(imap): remove Session from Imap structure

Connection establishment now happens only in one place in each IMAP loop.
Now all connection establishment happens in one place
and is limited by the ratelimit.

Backoff was removed from fake_idle
as it does not establish connections anymore.
If connection fails, fake_idle will return an error.
We then drop the connection and get back to the beginning of IMAP
loop.

Backoff may be still nice to have to delay retries
in case of constant connection failures
so we don't immediately hit ratelimit if the network is unusable
and returns immediate error on each connection attempt
(e.g. ICMP network unreachable error),
but adding backoff for connection failures is out of scope for this change.

src/download.rs

@@ -9,7 +9,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::config::Config;
 use crate::context::Context;
-use crate::imap::{Imap, ImapActionResult};
+use crate::imap::{session::Session, ImapActionResult};
 use crate::message::{Message, MsgId, Viewtype};
 use crate::mimeparser::{MimeMessage, Part};
 use crate::tools::time;
@@ -129,9 +129,11 @@ impl Message {
 /// Actually download a message partially downloaded before.
 ///
 /// Most messages are downloaded automatically on fetch instead.
-pub(crate) async fn download_msg(context: &Context, msg_id: MsgId, imap: &mut Imap) -> Result<()> {
-    imap.prepare(context).await?;
-
+pub(crate) async fn download_msg(
+    context: &Context,
+    msg_id: MsgId,
+    session: &mut Session,
+) -> Result<()> {
     let msg = Message::load_from_db(context, msg_id).await?;
     let row = context
         .sql
@@ -152,7 +154,7 @@ pub(crate) async fn download_msg(context: &Context, msg_id: MsgId, imap: &mut Im
         return Err(anyhow!("Call download_full() again to try over."));
     };
 
-    match imap
+    match session
         .fetch_single_msg(
             context,
             &server_folder,
@@ -169,7 +171,7 @@ pub(crate) async fn download_msg(context: &Context, msg_id: MsgId, imap: &mut Im
     }
 }
 
-impl Imap {
+impl Session {
     /// Download a single message and pipe it to receive_imf().
     ///
     /// receive_imf() is not directly aware that this is a result of a call to download_msg(),
@@ -194,10 +196,7 @@ impl Imap {
 
         let mut uid_message_ids: BTreeMap<u32, String> = BTreeMap::new();
         uid_message_ids.insert(uid, rfc724_mid);
-        let Some(session) = self.session.as_mut() else {
-            return ImapActionResult::Failed;
-        };
-        let (last_uid, _received) = match session
+        let (last_uid, _received) = match self
             .fetch_many_msgs(
                 context,
                 folder,