.Net Agents - Make messages parameter optional for `OpenAIAssistant…

…Agent.InvokeStreamingAsync` (#8915)

### Motivation and Context
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Customer input identified that `messages` parameter may not always be
required by the caller.

Fixes: #8857

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

Providing the complete messages is a convenience and shouldn't be
required.

### 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/OpenAIAssistant_Streaming.cs

@@ -83,7 +83,7 @@ private async Task InvokeAgentAsync(OpenAIAssistantAgent agent, string threadId,
         ChatHistory history = [];
 
         bool isFirst = false;
-        await foreach (StreamingChatMessageContent response in agent.InvokeStreamingAsync(threadId, history))
+        await foreach (StreamingChatMessageContent response in agent.InvokeStreamingAsync(threadId, messages: history))
         {
             if (string.IsNullOrEmpty(response.Content))
             {

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

@@ -348,7 +348,7 @@ public static async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamin
         OpenAIAssistantAgent agent,
         AssistantClient client,
         string threadId,
-        IList<ChatMessageContent> messages,
+        IList<ChatMessageContent>? messages,
         OpenAIAssistantInvocationOptions? invocationOptions,
         ILogger logger,
         Kernel kernel,
@@ -413,7 +413,7 @@ public static async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamin
                 if (functionCalls.Length > 0)
                 {
                     // Emit function-call content
-                    messages.Add(GenerateFunctionCallContent(agent.GetName(), functionCalls));
+                    messages?.Add(GenerateFunctionCallContent(agent.GetName(), functionCalls));
 
                     // Invoke functions for each tool-step
                     IEnumerable<Task<FunctionResultContent>> functionResultTasks = ExecuteFunctionSteps(agent, functionCalls, cancellationToken);
@@ -425,7 +425,7 @@ public static async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamin
                     ToolOutput[] toolOutputs = GenerateToolOutputs(functionResults);
                     asyncUpdates = client.SubmitToolOutputsToRunStreamingAsync(run, toolOutputs);
 
-                    messages.Add(GenerateFunctionResultsContent(agent.GetName(), functionResults));
+                    messages?.Add(GenerateFunctionResultsContent(agent.GetName(), functionResults));
                 }
             }
 
@@ -440,7 +440,7 @@ public static async IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamin
                     if (message != null)
                     {
                         ChatMessageContent content = GenerateMessageContent(agent.GetName(), message);
-                        messages.Add(content);
+                        messages?.Add(content);
                     }
                 }
 

dotnet/src/Agents/OpenAI/OpenAIAssistantAgent.cs

@@ -298,41 +298,41 @@ public async IAsyncEnumerable<ChatMessageContent> InvokeAsync(
     /// Invoke the assistant on the specified thread.
     /// </summary>
     /// <param name="threadId">The thread identifier</param>
-    /// <param name="messages">The receiver for the completed messages generated</param>
     /// <param name="arguments">Optional arguments to pass to the agents's invocation, including any <see cref="PromptExecutionSettings"/>.</param>
     /// <param name="kernel">The <see cref="Kernel"/> containing services, plugins, and other state for use by the agent.</param>
+    /// <param name="messages">Optional receiver of the completed messages generated</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>Asynchronous enumeration of messages.</returns>
     /// <remarks>
     /// The `arguments` parameter is not currently used by the agent, but is provided for future extensibility.
     /// </remarks>
     public IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamingAsync(
         string threadId,
-        ChatHistory messages,
         KernelArguments? arguments = null,
         Kernel? kernel = null,
+        ChatHistory? messages = null,
         CancellationToken cancellationToken = default)
-            => this.InvokeStreamingAsync(threadId, messages, options: null, arguments, kernel, cancellationToken);
+            => this.InvokeStreamingAsync(threadId, options: null, arguments, kernel, messages, cancellationToken);
 
     /// <summary>
     /// Invoke the assistant on the specified thread.
     /// </summary>
     /// <param name="threadId">The thread identifier</param>
-    /// <param name="messages">The receiver for the completed messages generated</param>
     /// <param name="options">Optional invocation options</param>
     /// <param name="arguments">Optional arguments to pass to the agents's invocation, including any <see cref="PromptExecutionSettings"/>.</param>
     /// <param name="kernel">The <see cref="Kernel"/> containing services, plugins, and other state for use by the agent.</param>
+    /// <param name="messages">Optional receiver of the completed messages generated</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>Asynchronous enumeration of messages.</returns>
     /// <remarks>
     /// The `arguments` parameter is not currently used by the agent, but is provided for future extensibility.
     /// </remarks>
     public IAsyncEnumerable<StreamingChatMessageContent> InvokeStreamingAsync(
         string threadId,
-        ChatHistory messages,
         OpenAIAssistantInvocationOptions? options,
         KernelArguments? arguments = null,
         Kernel? kernel = null,
+        ChatHistory? messages = null,
         CancellationToken cancellationToken = default)
     {
         this.ThrowIfDeleted();