Minor improvements (QuantConnect#8223)

- Liquidate takes into account invested flag
- Minor tweaks for symbol representation parsing

Algorithm/QCAlgorithm.Trading.cs

@@ -1244,11 +1244,19 @@ public List<OrderTicket> Liquidate(IEnumerable<Symbol> symbols, bool asynchronou
                 var orders = Transactions.GetOpenOrders(symbolToLiquidate);
 
                 // get quantity in portfolio
-                var quantity = Portfolio[symbolToLiquidate].Quantity;
+                var quantity = 0m;
+                var holdings = Portfolio[symbolToLiquidate];
+                if (holdings.Invested)
+                {
+                    // invested flag might filter some quantity that's less than lot size
+                    quantity = holdings.Quantity;
+                }
 
                 // if there is only one open market order that would close the position, do nothing
                 if (orders.Count == 1 && quantity != 0 && orders[0].Quantity == -quantity && orders[0].Type == OrderType.Market)
+                {
                     continue;
+                }
 
                 // cancel all open orders
                 var marketOrdersQuantity = 0m;

Common/SymbolRepresentation.cs

@@ -373,15 +373,14 @@ public static Symbol ParseOptionTickerOSI(string ticker, SecurityType securityTy
         /// <returns>Symbol object for the specified OSI option ticker string</returns>
         public static Symbol ParseOptionTickerOSI(string ticker, SecurityType securityType, OptionStyle optionStyle, string market)
         {
-
             var match = _optionTickerRegex.Match(ticker);
             if (!match.Success)
             {
-                throw new FormatException("Invalid ticker format.");
+                throw new FormatException($"Invalid ticker format {ticker}");
             }
 
             var optionTicker = match.Groups[1].Value;
-            var expiration = DateTime.ParseExact(match.Groups[2].Value, "yyMMdd", null);
+            var expiration = DateTime.ParseExact(match.Groups[2].Value, DateFormat.SixCharacter, null);
             OptionRight right = match.Groups[3].Value.ToUpper() == "C" ? OptionRight.Call : OptionRight.Put;
             var strike = decimal.Parse(match.Groups[4].Value) / 1000m;