Adding HandlebarsPlanCreationException

[teresaqhoang]

Motivation and Context

Resolves #4440

This PR adds a more detailed HandlebarsPlanCreationException type, which allows users to inspect the prompt and model results, if available.

Als includes a minor update to sample to add clarifying comments.

Description

Users can catch HandlebarsPlanCreationException to inspect the prompt, proposed plan, and exception details on error. All exceptions will be bubbled up to the caller.

var planner = new HandlebarsPlanner();

try
{
     var plan = await planner.CreatePlanAsync(kernel, intent, cancellationToken);
}
catch (HandlebarsPlanCreationException ex)
{
     Console.WriteLine(ex.Message);
     Console.WriteLine(ex.InnerException?.Message);
     Console.WriteLine($"CreatePlan Prompt: {ex.CreatePlanPrompt}");
     Console.WriteLine($"Proposed plan (model output): {ex.ModelResults.Content}");
     throw ex;
}

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Krzysztof318]

But isn't this a return to how kernel errors were once handled? ADR 0004 describes a departure from such solution in favor of throwing exceptions.

[teresaqhoang]

But isn't this a return to how kernel errors were once handled? ADR 0004 describes a departure from such solution in favor of throwing exceptions.

Great callout @Krzysztof318, that slipped my mind. Pivoted to creating a narrower exception type containing these details.