chore: import upstream snapshot with attribution

Algorithm.CSharp/IndexSecurityIsNotTradableRegressionAlgorithm.cs

@@ -0,0 +1,27 @@
+/*
+ * QUANTCONNECT.COM - Democratizing Finance, Empowering Individuals.
+ * Lean Algorithmic Trading Engine v2.0. Copyright 2014 QuantConnect Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+*/
+
+namespace QuantConnect.Algorithm.CSharp
+{
+    /// <summary>
+    /// Regression algorithm to test SignalExportManager still works as expected even when index securities
+    /// are not marked as tradable
+    /// </summary>
+    public class IndexSecurityIsNotTradableRegressionAlgorithm: IndexSecurityCanBeTradableRegressionAlgorithm
+    {
+        public override bool IsTradable { get; set; }
+    }
+}