.Net: Respect custom ToolChoice and ParallelToolCallsEnabled options in OpenAIResponseAgent (#13275)

### Motivation and Context

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Fixes #13253 by respecting custom `ToolChoice` and
`ParallelToolCallsEnabled` options in `OpenAIResponseAgent`.

### Description

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

Previously, when creating `ResponseCreationOptions` for
`OpenAIResponseAgent` with kernel plugins, `ToolChoice` will be set to
`Auto` and `ParallelToolCallsEnabled` will be set to `true` regardless
of the options provided when creating the agent.

This change fixes the problem by setting the default options only when
they are not provided, respecting the custom options.

### 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 😄

---------

Co-authored-by: Jiaming Liang <[email protected]>
Co-authored-by: Mark Wallace <[email protected]>
Co-authored-by: Roger Barreto <[email protected]>

Author: 4 people

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

@@ -60,8 +60,10 @@ internal static ResponseCreationOptions CreateOptions(
         if (responseTools is not null && responseTools.Any())
         {
             creationOptions.Tools.AddRange(responseTools);
-            creationOptions.ToolChoice = ResponseToolChoice.CreateAutoChoice();
-            creationOptions.ParallelToolCallsEnabled = true;
+            if (creationOptions.ToolChoice is null)
+            {
+                creationOptions.ToolChoice = ResponseToolChoice.CreateAutoChoice();
+            }
         }
 
         return creationOptions;

dotnet/src/Agents/UnitTests/OpenAI/Internal/ResponseCreationOptionsFactoryTests.cs

@@ -261,6 +261,60 @@ public void CreateOptionsWithNullAgentNameTest()
         Assert.Equal("UnnamedAgent", options.EndUserId); // Null name should fallback to "UnnamedAgent"
     }
 
+    /// <summary>
+    /// Verify response options creation with kernel plugin and default response options.
+    /// </summary>
+    [Fact]
+    public void CreateOptionsWithKernelPluginAndDefaultOptionsTest()
+    {
+        // Arrange
+        var mockAgent = CreateMockAgent("Test Agent", "You are a helpful assistant.", storeEnabled: false);
+        var mockThread = CreateMockAgentThread(null);
+        mockAgent.Kernel.Plugins.AddFromObject(new TestPlugin());
+
+        // Act
+        var options = ResponseCreationOptionsFactory.CreateOptions(mockAgent, mockThread.Object, null);
+
+        // Assert
+        Assert.NotNull(options);
+        Assert.Single(options.Tools);
+        Assert.NotNull(options.ToolChoice);
+        Assert.Equal(ResponseToolChoiceKind.Auto, options.ToolChoice.Kind);
+        Assert.Null(options.ParallelToolCallsEnabled);
+    }
+
+    /// <summary>
+    /// Verify response options creation with kernel plugin and custom response options.
+    /// </summary>
+    [Fact]
+    public void CreateOptionsWithKernelPluginAndCustomOptionsTest()
+    {
+        // Arrange
+        var mockAgent = CreateMockAgent("Test Agent", "You are a helpful assistant.", storeEnabled: false);
+        var mockThread = CreateMockAgentThread(null);
+        mockAgent.Kernel.Plugins.AddFromObject(new TestPlugin());
+
+        // Custom invoke options should be respected
+        var invokeOptions = new OpenAIResponseAgentInvokeOptions
+        {
+            ResponseCreationOptions = new ResponseCreationOptions
+            {
+                ToolChoice = ResponseToolChoice.CreateNoneChoice(),
+                ParallelToolCallsEnabled = false
+            }
+        };
+
+        // Act
+        var options = ResponseCreationOptionsFactory.CreateOptions(mockAgent, mockThread.Object, invokeOptions);
+
+        // Assert
+        Assert.NotNull(options);
+        Assert.Single(options.Tools);
+        Assert.NotNull(options.ToolChoice);
+        Assert.Equal(ResponseToolChoiceKind.None, options.ToolChoice.Kind);
+        Assert.False(options.ParallelToolCallsEnabled);
+    }
+
     private static OpenAIResponseAgent CreateMockAgent(string? name, string? instructions, bool storeEnabled)
     {
         var mockClient = new Mock<OpenAIResponseClient>();
@@ -281,4 +335,11 @@ private static Mock<AgentThread> CreateMockAgentThread(string? threadId)
         mockThread.Setup(t => t.Id).Returns(threadId);
         return mockThread;
     }
+
+    private sealed class TestPlugin
+    {
+        [KernelFunction]
+        public void TestFunction()
+        { }
+    }
 }