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/types.rs

@@ -174,3 +174,20 @@ impl Transfer {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // A truncated TransferRequest from an untrusted peer must parse to defaults
+    // rather than panic (the read_* primitives are bounds-checked).
+    #[test]
+    fn transfer_new_from_truncated_message_does_not_panic() {
+        let mut message = Message::new_with_data(vec![1, 0, 0]);
+        let transfer = Transfer::new_from_message(&mut message);
+        assert_eq!(transfer.direction, 0);
+        assert_eq!(transfer.token, 0);
+        assert_eq!(transfer.filename, "");
+        assert_eq!(transfer.size, 0);
+    }
+}