fix(downloads): a download that hangs up mid-transfer is not an error at the egress gate

Seen live: yt-dlp closes a connection once it has read what it needed, and the
gate printed a traceback for each. A reset ends the tunnel quietly; anything
else the gate cannot finish is a log line with its code, not bare stderr.

backend/tests/test_egress.py

@@ -10,7 +10,9 @@ import http.server
 import ipaddress
 import socket
 import socketserver
+import struct
 import threading
+import time
 from pathlib import Path
 
 import pytest
@@ -178,6 +180,24 @@ def test_a_tunnel_into_the_network_is_refused_before_it_is_dialled(gate):
         internal.close()
 
 
+def test_a_client_that_hangs_up_mid_transfer_is_not_an_error(gate, monkeypatch):
+    # yt-dlp closes a connection once it has read what it needed; seen live as a traceback the gate
+    # printed for every such download.
+    errors_seen = []
+    monkeypatch.setattr(gate, "handle_error", lambda *a: errors_seen.append(a))
+    public = _public_server({"/big": (200, {}, b"x" * 4_000_000)})
+    try:
+        s = socket.create_connection(("127.0.0.1", gate.server_address[1]))
+        s.sendall(f"GET http://{PUBLIC}:{public.server_address[1]}/big HTTP/1.1\r\n\r\n".encode())
+        s.recv(1024)
+        s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack("ii", 1, 0))
+        s.close()  # a reset, not a polite close
+        time.sleep(0.5)
+        assert errors_seen == []
+    finally:
+        public.shutdown()
+
+
 # --- a real yt-dlp through it ----------------------------------------------------------------------