fix: reject malformed peer connection types instead of panicking

Peer::new_from_message, NewPeer::new_from_message and the listener's
parse_peer_init_message parsed an untrusted connection-type string with
.parse().unwrap(), panicking on anything other than P/F/D. Reachable from:
- a malicious server via ConnectToPeer (kills a thread-pool worker), and
- any peer connecting to our listener (kills the accept loop).

new_from_message now returns Option and callers drop malformed messages;
the listener rejects the connection. Added a regression test for truncated
TransferRequest parsing, now safe via the bounds-checked readers.

soulseek-rs-lib/src/message/server/connect_to_peer.rs

@@ -9,7 +9,9 @@ impl MessageHandler<ServerMessage> for ConnectToPeerHandler {
         18
     }
     fn handle(&self, message: &mut Message, sender: Sender<ServerMessage>) {
-        let peer = Peer::new_from_message(message);
-        sender.send(ServerMessage::ConnectToPeer(peer)).unwrap();
+        let Some(peer) = Peer::new_from_message(message) else {
+            return;
+        };
+        let _ = sender.send(ServerMessage::ConnectToPeer(peer));
     }
 }