.Net: Add unit test to confirm 429 exception handling (#9772)

### Motivation and Context

Issue #9666 

### Description

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### Contribution Checklist

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- [ ] 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 😄

dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromPromptTests.cs

@@ -3,6 +3,8 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Net;
+using System.Net.Http;
 using System.Runtime.CompilerServices;
 using System.Text.Json;
 using System.Threading;
@@ -843,6 +845,27 @@ public async Task InvokePromptAsyncWithChatCompletionReturnsMultipleResultsAsync
         }
     }
 
+    [Fact]
+    public async Task InvokePromptAsyncWithChatCompletionPropagatesTooManyRequestsAsync()
+    {
+        // Arrange
+        using var messageHandlerStub = new HttpMessageHandlerStub();
+        using var response = new HttpResponseMessage(System.Net.HttpStatusCode.TooManyRequests);
+        messageHandlerStub.ResponseToReturn = response;
+        using var httpClient = new HttpClient(messageHandlerStub, false);
+        var chatCompletion = new OpenAIChatCompletionService(modelId: "any", apiKey: "any", httpClient: httpClient);
+
+        KernelBuilder builder = new();
+        builder.Services.AddTransient<IChatCompletionService>((sp) => chatCompletion);
+        Kernel kernel = builder.Build();
+
+        // Act
+        var exception = await Assert.ThrowsAsync<HttpOperationException>(async () => await kernel.InvokePromptAsync("Prompt"));
+
+        // Assert
+        Assert.Equal(HttpStatusCode.TooManyRequests, exception.StatusCode);
+    }
+
     [Fact]
     public async Task InvokePromptAsyncWithPromptFunctionInTemplateAndSingleResultAsync()
     {