Fix format issues: file encoding and remove unused using directives

Co-authored-by: rogerbarreto <19890735+rogerbarreto@users.noreply.github.com>

Author: Copilot

dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/ChatClient/AsyncStreamingChatCompletionUpdateCollectionResultTests.cs

@@ -1,8 +1,7 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
 using System.ClientModel;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
 using OpenAI.Chat;

dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/ChatClient/AsyncStreamingResponseUpdateCollectionResultTests.cs

@@ -1,9 +1,8 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
 using System;
 using System.ClientModel;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
 using OpenAI.Responses;

dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/ChatClient/StreamingUpdatePipelineResponseTests.cs

@@ -1,9 +1,8 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
 using System;
 using System.ClientModel.Primitives;
 using System.Collections.Generic;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace Microsoft.Agents.AI.OpenAI.UnitTests.ChatClient;

dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/AgentResponseExtensionsTests.cs

@@ -1,7 +1,6 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
 using System;
-using Microsoft.Extensions.AI;
 using OpenAI.Chat;
 using ChatMessage = Microsoft.Extensions.AI.ChatMessage;
 using ChatRole = Microsoft.Extensions.AI.ChatRole;

dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIAssistantClientExtensionsTests.cs

@@ -845,7 +845,7 @@ public void AsAIAgent_LegacyOverload_WithNullClientResult_ThrowsArgumentNullExce
 
         // Act & Assert
         var exception = Assert.Throws<ArgumentNullException>(() =>
-            assistantClient.AsAIAgent((ClientResult<Assistant>)null!));
+            assistantClient.AsAIAgent(null!));
 
         Assert.Equal("assistantClientResult", exception.ParamName);
     }