.Net: Function Calling Planner: Catch exceptions and output error messages to chat history (#4455)

### Motivation and Context

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Resolves #3224. Exceptions thrown by invoked functions are currently not
being caught by the planner.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->
This change catches any non-critical exceptions from function
invocation, adds the error message into chat history, and lets the
planner retry.

### 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 😄

dotnet/src/IntegrationTests/Fakes/ThrowingEmailPluginFake.cs

@@ -0,0 +1,39 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.ComponentModel;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.SemanticKernel;
+
+#pragma warning disable CA1812 // Uninstantiated internal types
+
+namespace SemanticKernel.IntegrationTests.Fakes;
+
+internal sealed class ThrowingEmailPluginFake
+{
+    [KernelFunction, Description("Given an email address and message body, send an email")]
+    public Task<string> SendEmailAsync(
+        [Description("The body of the email message to send.")] string input = "",
+        [Description("The email address to send email to.")] string? email_address = "[email protected]")
+    {
+        // Throw a non-critical exception for testing
+        throw new ArgumentException($"Failed to send email to {email_address}");
+    }
+
+    [KernelFunction, Description("Lookup an email address for a person given a name")]
+    public Task<string> GetEmailAddressAsync(
+        ILogger logger,
+        [Description("The name of the person to email.")] string? input = null)
+    {
+        // Throw a critical exception for testing
+        throw new InvalidProgramException();
+    }
+
+    [KernelFunction, Description("Write a short poem for an e-mail")]
+    public Task<string> WritePoemAsync(
+        [Description("The topic of the poem.")] string input)
+    {
+        return Task.FromResult($"Roses are red, violets are blue, {input} is hard, so is this test.");
+    }
+}

dotnet/src/IntegrationTests/Planners/Stepwise/FunctionCallingStepwisePlannerTests.cs

@@ -72,6 +72,42 @@ public sealed class FunctionCallingStepwisePlannerTests : IDisposable
         }
     }
 
+    [Fact]
+    public async Task DoesNotThrowWhenPluginFunctionThrowsNonCriticalExceptionAsync()
+    {
+        Kernel kernel = this.InitializeKernel();
+        kernel.ImportPluginFromType<ThrowingEmailPluginFake>("Email");
+
+        var planner = new FunctionCallingStepwisePlanner(
+            new FunctionCallingStepwisePlannerConfig() { MaxIterations = 5 });
+
+        // Act
+        var planResult = await planner.ExecuteAsync(kernel, "Email a poem about cats to [email protected]");
+
+        // Assert - should contain the expected answer & function calls within the maximum iterations
+        Assert.NotNull(planResult);
+        Assert.True(planResult.Iterations > 0);
+        Assert.True(planResult.Iterations <= 5);
+
+        string serializedChatHistory = JsonSerializer.Serialize(planResult.ChatHistory);
+        Assert.Contains("Email_WritePoem", serializedChatHistory, StringComparison.InvariantCultureIgnoreCase);
+        Assert.Contains("Email_SendEmail", serializedChatHistory, StringComparison.InvariantCultureIgnoreCase);
+    }
+
+    [Fact]
+    public async Task ThrowsWhenPluginFunctionThrowsCriticalExceptionAsync()
+    {
+        Kernel kernel = this.InitializeKernel();
+        kernel.ImportPluginFromType<ThrowingEmailPluginFake>("Email");
+
+        var planner = new FunctionCallingStepwisePlanner(
+            new FunctionCallingStepwisePlannerConfig() { MaxIterations = 5 });
+
+        // Act & Assert
+        // Planner should call ThrowingEmailPluginFake.GetEmailAddressAsync, which throws InvalidProgramException
+        await Assert.ThrowsAsync<InvalidProgramException>(async () => await planner.ExecuteAsync(kernel, "What is Kelly's email address?"));
+    }
+
     private Kernel InitializeKernel()
     {
         OpenAIConfiguration? openAIConfiguration = this._configuration.GetSection("Planners:OpenAI").Get<OpenAIConfiguration>();

dotnet/src/Planners/Planners.OpenAI/Stepwise/FunctionCallingStepwisePlanner.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Diagnostics.CodeAnalysis;
@@ -135,8 +136,9 @@ public sealed class FunctionCallingStepwisePlanner
                         var result = (await clonedKernel.InvokeAsync(pluginFunction, arguments, cancellationToken).ConfigureAwait(false)).GetValue<object>();
                         chatHistoryForSteps.AddMessage(AuthorRole.Tool, ParseObjectAsString(result), metadata: new Dictionary<string, object?>(1) { { OpenAIChatMessageContent.ToolIdProperty, functionResponse.Id } });
                     }
-                    catch (KernelException)
+                    catch (Exception ex) when (!ex.IsCriticalException())
                     {
+                        chatHistoryForSteps.AddMessage(AuthorRole.Tool, ex.Message, metadata: new Dictionary<string, object?>(1) { { OpenAIChatMessageContent.ToolIdProperty, functionResponse.Id } });
                         chatHistoryForSteps.AddUserMessage($"Failed to execute function {functionResponse.FullyQualifiedName}. Try something else!");
                     }
                 }