Add IndexOption support to TerminalLinkBrokerageModel (#9633)

Matches Lean.Brokerages.TerminalLink#131, which maps IndexOption
contracts to the Bloomberg Index yellow key for EMSX order routing.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: AlexCatarino

Common/Brokerages/TerminalLinkBrokerageModel.cs

@@ -28,6 +28,7 @@ public class TerminalLinkBrokerageModel : DefaultBrokerageModel
         {
             SecurityType.Equity,
             SecurityType.Option,
+            SecurityType.IndexOption,
             SecurityType.Future,
         };
 

Tests/Common/Brokerages/TerminalLinkBrokerageModelTests.cs

@@ -29,6 +29,7 @@ public class TerminalLinkBrokerageModelTests
 
         [TestCase("SPY", SecurityType.Equity)]
         [TestCase("SPY", SecurityType.Option)]
+        [TestCase("SPX", SecurityType.IndexOption)]
         [TestCase("ES", SecurityType.Future)]
         public void CanSubmitOrder_ForSupportedSecurityTypes(string ticker, SecurityType securityType)
         {
@@ -40,8 +41,8 @@ public void CanSubmitOrder_ForSupportedSecurityTypes(string ticker, SecurityType
             Assert.IsNull(message);
         }
 
-        // Index is data-only on TerminalLink; Forex/Crypto/Cfd and option chains other than
-        // equity options (IndexOption/FutureOption) are not supported for trading.
+        // Index is data-only on TerminalLink; Forex/Crypto/Cfd and FutureOption
+        // are not supported for trading.
         [TestCase("EURUSD", SecurityType.Forex)]
         [TestCase("BTCUSD", SecurityType.Crypto)]
         [TestCase("DE10YBEUR", SecurityType.Cfd)]