.Net: Change ChatCompletionAgent to notify intermediate messages as soon as they are available. (#12575)

### Motivation and Context

Addresses #12521

### Contribution Checklist

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

- [ ] The code builds clean without any errors or warnings
- [ ] 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
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Author: westey-m

dotnet/src/Agents/Core/ChatCompletionAgent.cs

@@ -391,6 +391,7 @@ private async IAsyncEnumerable<StreamingChatMessageContent> InternalInvokeStream
 
         this.Logger.LogAgentChatServiceInvokedStreamingAgent(nameof(InvokeAsync), this.Id, agentName, serviceType);
 
+        int messageIndex = messageCount;
         AuthorRole? role = null;
         StringBuilder builder = new();
         await foreach (StreamingChatMessageContent message in messages.ConfigureAwait(false))
@@ -401,18 +402,18 @@ private async IAsyncEnumerable<StreamingChatMessageContent> InternalInvokeStream
 
             builder.Append(message.ToString());
 
-            yield return message;
-        }
+            // Capture mutated messages related function calling / tools
+            for (; messageIndex < chat.Count; messageIndex++)
+            {
+                ChatMessageContent chatMessage = chat[messageIndex];
 
-        // Capture mutated messages related function calling / tools
-        for (int messageIndex = messageCount; messageIndex < chat.Count; messageIndex++)
-        {
-            ChatMessageContent message = chat[messageIndex];
+                chatMessage.AuthorName = this.Name;
 
-            message.AuthorName = this.Name;
+                await onNewMessage(chatMessage).ConfigureAwait(false);
+                history.Add(chatMessage);
+            }
 
-            await onNewMessage(message).ConfigureAwait(false);
-            history.Add(message);
+            yield return message;
         }
 
         // Do not duplicate terminated function result to history