.Net Agents - Protect against null-reference for Assistant file-search result when streaming (#9742)

### Motivation and Context
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Fixes: #9723

Must distinguish between file-search result and function-call result for
assistant streaming without triggering `NullReferenceException`

### Description
<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

A file-search result provide any streaming result; rather, it results in
an annotation on the assistant message. Still it must be distinguished
from a function-call result.

Since `RunStepDetailsUpdate.FunctionName` property throws a
`NullReferenceException` when being evaluated, using
`RunStepDetailsUpdate.FunctionResult` property.

Merged fix to Open AI SDK:
openai/openai-dotnet#293


### Contribution Checklist
<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: crickman

dotnet/src/Agents/OpenAI/Internal/AssistantThreadActions.cs

@@ -451,7 +451,7 @@ public static async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamin
                     {
                         yield return toolContent;
                     }
-                    else
+                    else if (detailsUpdate.FunctionOutput != null)
                     {
                         yield return
                             new StreamingChatMessageContent(AuthorRole.Assistant, null)

dotnet/src/IntegrationTests/Agents/OpenAIAssistantAgentTests.cs

@@ -2,6 +2,7 @@
 using System;
 using System.ClientModel;
 using System.ComponentModel;
+using System.IO;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
@@ -11,6 +12,8 @@
 using Microsoft.SemanticKernel.Agents;
 using Microsoft.SemanticKernel.Agents.OpenAI;
 using Microsoft.SemanticKernel.ChatCompletion;
+using OpenAI.Files;
+using OpenAI.VectorStores;
 using SemanticKernel.IntegrationTests.TestSettings;
 using xRetry;
 using Xunit;
@@ -216,6 +219,56 @@ await OpenAIAssistantAgent.CreateAsync(
         finally
         {
             await agent.DeleteThreadAsync(threadId);
+            await agent.DeleteAsync();
+        }
+    }
+
+    /// <summary>
+    /// Integration test for <see cref="OpenAIAssistantAgent"/> using function calling
+    /// and targeting Open AI services.
+    /// </summary>
+    [Fact]
+    public async Task AzureOpenAIAssistantAgentStreamingFileSearchAsync()
+    {
+        var azureOpenAIConfiguration = this._configuration.GetSection("AzureOpenAI").Get<AzureOpenAIConfiguration>();
+        Assert.NotNull(azureOpenAIConfiguration);
+
+        OpenAIClientProvider provider = OpenAIClientProvider.ForAzureOpenAI(new AzureCliCredential(), new Uri(azureOpenAIConfiguration.Endpoint));
+        OpenAIAssistantAgent agent =
+            await OpenAIAssistantAgent.CreateAsync(
+                provider,
+                new(azureOpenAIConfiguration.ChatDeploymentName!),
+                new Kernel());
+
+        // Upload file - Using a table of fictional employees.
+        OpenAIFileClient fileClient = provider.Client.GetOpenAIFileClient();
+        await using Stream stream = File.OpenRead("TestData/employees.pdf")!;
+        OpenAIFile fileInfo = await fileClient.UploadFileAsync(stream, "employees.pdf", FileUploadPurpose.Assistants);
+
+        // Create a vector-store
+        VectorStoreClient vectorStoreClient = provider.Client.GetVectorStoreClient();
+        CreateVectorStoreOperation result =
+            await vectorStoreClient.CreateVectorStoreAsync(waitUntilCompleted: false,
+                new VectorStoreCreationOptions()
+                {
+                    FileIds = { fileInfo.Id }
+                });
+
+        string threadId = await agent.CreateThreadAsync();
+        try
+        {
+            await agent.AddChatMessageAsync(threadId, new(AuthorRole.User, "Who works in sales?"));
+            ChatHistory messages = [];
+            var chunks = await agent.InvokeStreamingAsync(threadId, messages: messages).ToArrayAsync();
+            Assert.NotEmpty(chunks);
+            Assert.Single(messages);
+        }
+        finally
+        {
+            await agent.DeleteThreadAsync(threadId);
+            await agent.DeleteAsync();
+            await vectorStoreClient.DeleteVectorStoreAsync(result.VectorStoreId);
+            await fileClient.DeleteFileAsync(fileInfo.Id);
         }
     }
 

dotnet/src/IntegrationTests/IntegrationTests.csproj

@@ -1,11 +1,11 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <AssemblyName>IntegrationTests</AssemblyName>
     <RootNamespace>SemanticKernel.IntegrationTests</RootNamespace>
     <TargetFramework>net8.0</TargetFramework>
     <IsTestProject>true</IsTestProject>
     <IsPackable>false</IsPackable>
-    <NoWarn>$(NoWarn);CA2007,CA1861,VSTHRD111,SKEXP0001,SKEXP0010,SKEXP0020,SKEXP0040,SKEXP0050,SKEXP0060,SKEXP0070,SKEXP0080,SKEXP0110</NoWarn>
+    <NoWarn>$(NoWarn);CA2007,CA1861,VSTHRD111,SKEXP0001,SKEXP0010,SKEXP0020,SKEXP0040,SKEXP0050,SKEXP0060,SKEXP0070,SKEXP0080,SKEXP0110,OPENAI001</NoWarn>
     <UserSecretsId>b7762d10-e29b-4bb1-8b74-b6d69a667dd4</UserSecretsId>
   </PropertyGroup>
   <ItemGroup>
@@ -39,7 +39,6 @@
     <None Remove="prompts\GenerateStoryHandlebars.yaml" />
     <None Remove="skills\FunSkill\Joke\config.json" />
     <None Remove="skills\FunSkill\Joke\skprompt.txt" />
-    <None Remove="TestData\semantic-kernel-info.txt" />
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Configuration" />
@@ -179,9 +178,6 @@
     <EmbeddedResource Include="skills\FunSkill\Joke\skprompt.txt">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </EmbeddedResource>
-    <EmbeddedResource Include="TestData\semantic-kernel-info.txt">
-      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
-    </EmbeddedResource>
   </ItemGroup>
 
   <ItemGroup>