Fix missing System using in AnthropicSkillsIntegrationTests

Add 'using System;' for InvalidOperationException in try/catch blocks.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: giles17

dotnet/tests/AnthropicChatCompletion.IntegrationTests/AnthropicSkillsIntegrationTests.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using System.Threading.Tasks;
 using AgentConformance.IntegrationTests.Support;
 using Anthropic;