.Net Agents - Emit streaming function call content for assistant agent (

#9359)

### Motivation and Context
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Expose `StreamingFunctionCallUpdateContent` for streaming output of
`OpenAIAssistantAgent`

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

@matthewbolanos identified assistant streaming wasn't aligned with
chat-completion in providing a message with
`StreamingFunctionCallUpdateContent` for a function-call.

### 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 😄

dotnet/samples/Concepts/Agents/ChatCompletion_Streaming.cs

@@ -81,6 +81,12 @@ private async Task InvokeAgentAsync(ChatCompletionAgent agent, ChatHistory chat,
         {
             if (string.IsNullOrEmpty(response.Content))
             {
+                StreamingFunctionCallUpdateContent? functionCall = response.Items.OfType<StreamingFunctionCallUpdateContent>().SingleOrDefault();
+                if (!string.IsNullOrEmpty(functionCall?.Name))
+                {
+                    Console.WriteLine($"\n# {response.Role} - {response.AuthorName ?? "*"}: FUNCTION CALL - {functionCall.Name}");
+                }
+
                 continue;
             }
 

dotnet/samples/Concepts/Agents/OpenAIAssistant_Streaming.cs

@@ -68,8 +68,8 @@ await OpenAIAssistantAgent.CreateAsync(
         string threadId = await agent.CreateThreadAsync(new OpenAIThreadCreationOptions { Metadata = AssistantSampleMetadata });
 
         // Respond to user input
-        await InvokeAgentAsync(agent, threadId, "What is the special soup?");
-        await InvokeAgentAsync(agent, threadId, "What is the special drink?");
+        await InvokeAgentAsync(agent, threadId, "What is the special soup and its price?");
+        await InvokeAgentAsync(agent, threadId, "What is the special drink and its price?");
 
         // Output the entire chat history
         await DisplayChatHistoryAsync(agent, threadId);
@@ -120,6 +120,12 @@ private async Task InvokeAgentAsync(OpenAIAssistantAgent agent, string threadId,
         {
             if (string.IsNullOrEmpty(response.Content))
             {
+                StreamingFunctionCallUpdateContent? functionCall = response.Items.OfType<StreamingFunctionCallUpdateContent>().SingleOrDefault();
+                if (functionCall != null)
+                {
+                    Console.WriteLine($"\n# {response.Role} - {response.AuthorName ?? "*"}: FUNCTION CALL - {functionCall.Name}");
+                }
+
                 continue;
             }
 

dotnet/samples/GettingStartedWithAgents/Step08_Assistant.cs

@@ -43,8 +43,8 @@ await OpenAIAssistantAgent.CreateAsync(
         try
         {
             await InvokeAgentAsync("Hello");
-            await InvokeAgentAsync("What is the special soup?");
-            await InvokeAgentAsync("What is the special drink?");
+            await InvokeAgentAsync("What is the special soup and its price?");
+            await InvokeAgentAsync("What is the special drink and its price?");
             await InvokeAgentAsync("Thank you");
         }
         finally

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

@@ -404,6 +404,15 @@ public static async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamin
                     {
                         yield return toolContent;
                     }
+                    else
+                    {
+                        yield return
+                            new StreamingChatMessageContent(AuthorRole.Assistant, null)
+                            {
+                                AuthorName = agent.Name,
+                                Items = [new StreamingFunctionCallUpdateContent(detailsUpdate.ToolCallId, detailsUpdate.FunctionName, detailsUpdate.FunctionArguments)]
+                            };
+                    }
                 }
                 else if (update is RunStepUpdate stepUpdate)
                 {
@@ -493,15 +502,16 @@ await RetrieveMessageAsync(
                     {
                         foreach (RunStepToolCall toolCall in step.Details.ToolCalls)
                         {
-                            switch (toolCall.ToolKind)
+                            if (toolCall.ToolKind == RunStepToolCallKind.Function)
+                            {
+                                messages?.Add(GenerateFunctionResultContent(agent.GetName(), stepFunctionResults[step.Id], step));
+                                stepFunctionResults.Remove(step.Id);
+                                break;
+                            }
+
+                            if (toolCall.ToolKind == RunStepToolCallKind.CodeInterpreter)
                             {
-                                case RunStepToolCallKind.CodeInterpreter:
-                                    messages?.Add(GenerateCodeInterpreterContent(agent.GetName(), toolCall.CodeInterpreterInput, step));
-                                    break;
-                                case RunStepToolCallKind.Function:
-                                    messages?.Add(GenerateFunctionResultContent(agent.GetName(), stepFunctionResults[step.Id], step));
-                                    stepFunctionResults.Remove(step.Id);
-                                    break;
+                                messages?.Add(GenerateCodeInterpreterContent(agent.GetName(), toolCall.CodeInterpreterInput, step));
                             }
                         }
                     }