.Net Fix Agent Orchestration: Fix message type (#12715)

### Motivation and Context
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Replace inadvertant use of GroupChatOrchestration `Result` message with
the expected `Magentic` message type.

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

MagenticOrchestration does not define a handler for
GroupChatOrchestration `Result` message since it was not intended to mix
message protocols.

### Contribution Checklist
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- [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)
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script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
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- [X] I didn't break anyone 😄

Author: crickman

dotnet/src/Agents/Magentic/MagenticManagerActor.cs

@@ -8,7 +8,6 @@
 using System.Threading.Tasks;
 using Microsoft.Extensions.Logging;
 using Microsoft.SemanticKernel.Agents.Orchestration;
-using Microsoft.SemanticKernel.Agents.Orchestration.GroupChat;
 using Microsoft.SemanticKernel.Agents.Runtime;
 using Microsoft.SemanticKernel.Agents.Runtime.Core;
 using Microsoft.SemanticKernel.ChatCompletion;

dotnet/src/Agents/Magentic/MagenticMessages.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
+using Microsoft.SemanticKernel.ChatCompletion;
 
 namespace Microsoft.SemanticKernel.Agents.Magentic;
 
@@ -76,4 +77,9 @@ public sealed class InputTask
     /// Extension method to convert a <see cref="ChatMessageContent"/> to a <see cref="Result"/> message.
     /// </summary>
     public static Result AsResultMessage(this ChatMessageContent message) => new() { Message = message };
+
+    /// <summary>
+    /// Extension method to convert a <see cref="string"/> to a <see cref="Result"/>.
+    /// </summary>
+    public static Result AsResultMessage(this string text) => new() { Message = new(AuthorRole.Assistant, text) };
 }

dotnet/src/Agents/Orchestration/GroupChat/GroupChatMessages.cs

@@ -8,7 +8,7 @@ namespace Microsoft.SemanticKernel.Agents.Orchestration.GroupChat;
 /// <summary>
 /// Common messages used for agent chat patterns.
 /// </summary>
-public static class GroupChatMessages
+internal static class GroupChatMessages
 {
     /// <summary>
     /// An empty message instance as a default.
@@ -79,7 +79,7 @@ public sealed class InputTask
     public static InputTask AsInputTaskMessage(this IEnumerable<ChatMessageContent> messages) => new() { Messages = messages };
 
     /// <summary>
-    /// Extension method to convert a <see cref="ChatMessageContent"/> to a <see cref="Result"/>.
+    /// Extension method to convert a <see cref="string"/> to a <see cref="Result"/>.
     /// </summary>
     public static Result AsResultMessage(this string text) => new() { Message = new(AuthorRole.Assistant, text) };
 }