fix(peer): handle UploadDenied so refused downloads fail fast

Peer message code 50 had no handler, so the dispatcher dropped it and a
download refused by the peer stayed Queued until the caller's timeout.

The new handler reads the filename and reason and routes the denial into
the same failure path as UploadFailed. A reason of "Queued" is status,
not a failure: some clients send it for a download that is still held in
their queue, and failing it would kill healthy downloads after a
reconnect, so that one is only logged.

Fixes #29

soulseek-rs-lib/src/message/peer/upload_denied.rs

@@ -0,0 +1,56 @@
+use crate::info;
+use crate::{
+    message::{Message, MessageHandler},
+    peer::PeerMessage,
+};
+use std::sync::mpsc::Sender;
+
+pub struct UploadDeniedHandler;
+impl MessageHandler<PeerMessage> for UploadDeniedHandler {
+    fn get_code(&self) -> u8 {
+        50
+    }
+    fn handle(&self, message: &mut Message, sender: Sender<PeerMessage>) {
+        let filename = message.read_string();
+        let reason = message.read_string();
+        info!("Upload denied for {}: {}", filename, reason);
+        if reason == "Queued" {
+            return;
+        }
+        let _ = sender.send(PeerMessage::UploadFailed(String::new(), filename));
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::message::framed;
+
+    #[test]
+    fn a_queued_reason_is_not_a_failure() {
+        let (tx, rx) = std::sync::mpsc::channel();
+        let mut message = framed(|m| {
+            m.write_string("@@share\\gone.mp3").write_string("Queued");
+        });
+
+        UploadDeniedHandler.handle(&mut message, tx);
+        assert!(rx.try_recv().is_err());
+    }
+
+    #[test]
+    fn the_denied_filename_is_forwarded() {
+        let (tx, rx) = std::sync::mpsc::channel();
+        let mut message = framed(|m| {
+            m.write_string("@@share\\gone.mp3")
+                .write_string("File not shared.");
+        });
+
+        UploadDeniedHandler.handle(&mut message, tx);
+        match rx.try_recv() {
+            Ok(PeerMessage::UploadFailed(_, filename)) => {
+                assert_eq!(filename, "@@share\\gone.mp3");
+            }
+            other => panic!("unexpected: {other:?}"),
+        }
+    }
+}