.Net Agents - Identify assistant code-interpreter response with metad…

…ata (#7882)

### Motivation and Context
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### Description
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Align with same pattern introduced in Python:
#7854

In response to customer input:
#7601


### Contribution Checklist
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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
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dotnet/src/Agents/OpenAI/AssistantThreadActions.cs

@@ -408,6 +408,7 @@ private static ChatMessageContent GenerateCodeInterpreterContent(string agentNam
                 ])
             {
                 AuthorName = agentName,
+                Metadata = new Dictionary<string, object?> { { OpenAIAssistantAgent.CodeInterpreterMetadataKey, true } },
             };
     }
 

dotnet/src/Agents/OpenAI/OpenAIAssistantAgent.cs

@@ -18,8 +18,13 @@ namespace Microsoft.SemanticKernel.Agents.OpenAI;
 /// <summary>
 /// A <see cref="KernelAgent"/> specialization based on Open AI Assistant / GPT.
 /// </summary>
-public sealed partial class OpenAIAssistantAgent : KernelAgent
+public sealed class OpenAIAssistantAgent : KernelAgent
 {
+    /// <summary>
+    /// Metadata key that identifies code-interpreter content.
+    /// </summary>
+    public const string CodeInterpreterMetadataKey = "code";
+
     private readonly Assistant _assistant;
     private readonly AssistantsClient _client;
     private readonly OpenAIAssistantConfiguration _config;