Keep the gateway connection alive and resume hosted queues after reconnecting

The client doesn't ping while it's sending, so when the proxy answers its frames itself (everything it
sends in a hosted room), IMVU's gateway saw an idle connection and closed it after about a minute. A
ping now goes to the gateway in place of each answered frame. And after a reconnect, hosted rooms'
queues are taken out of C2gOpenFloodgates and answered here (the gateway never answered them, so the
client gave up and closed the room).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/proxy/proxy_test.go

@@ -427,7 +427,8 @@ func TestConnectFiltersOutgoingMessages(t *testing.T) {
 	}
 	conn.Close()
 	got := <-received
-	want := append(send(1, "messages", "hi!"), send(3, "control", "x")...)
+	// The dropped message's place: a ping, so the gateway doesn't see an idle connection.
+	want := append(append(send(1, "messages", "hi!"), imq.EncodeFrame(imq.C2gPing, nil)...), send(3, "control", "x")...)
 	if !bytes.Equal(got, want) {
 		t.Fatalf("gateway got %q, want %q", got, want)
 	}