.Net Agents - Support role-override for ChatCompletionAgent (#10601)

### Motivation and Context
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The "O*" series deep-reasoning models from OpenAI do not support
`System` role. For certain versions, support was added for a `Developer`
role.

### Description
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These notes will help understanding how your code works. Thanks! -->

Allow `ChatCompletionAgent` make use of "O*" series models by supporting
`InstructionsRole` that may be optionally overriden.

### 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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Author: crickman

dotnet/src/Agents/Core/ChatCompletionAgent.cs

@@ -47,6 +47,17 @@ public ChatCompletionAgent(
         this.Template = templateFactory?.Create(templateConfig);
     }
 
+    /// <summary>
+    /// Gets the role used for the agent instructions.  Defaults to "system".
+    /// </summary>
+    /// <remarks>
+    /// Certain versions of "O*" series (deep reasoning) models require the instructions
+    /// to be provided as "developer" role.  Other versions support neither role and
+    /// an agent targeting such a model cannot provide instructions.  Agent functionality
+    /// will be dictated entirely by the provided plugins.
+    /// </remarks>
+    public AuthorRole InstructionsRole { get; init; } = AuthorRole.System;
+
     /// <inheritdoc/>
     public override IAsyncEnumerable<ChatMessageContent> InvokeAsync(
         ChatHistory history,
@@ -113,7 +124,7 @@ private async Task<ChatHistory> SetupAgentChatHistoryAsync(
 
         if (!string.IsNullOrWhiteSpace(instructions))
         {
-            chat.Add(new ChatMessageContent(AuthorRole.System, instructions) { AuthorName = this.Name });
+            chat.Add(new ChatMessageContent(this.InstructionsRole, instructions) { AuthorName = this.Name });
         }
 
         chat.AddRange(history);