.Net: Removing Insufficient functions error (#4983)

### Motivation and Context

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This PR removes the InsufficientFunctions error message. If the planner
doesn't have enough context to create a plan, it should forego a
template altogether and return an error string. This will be captured by
the `InvalidTemplate` check.

### Description

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

Addresses #4442, which identified that this error handling was so
restrictive for most plan types.

### 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/Planners/Planners.Handlebars/Handlebars/CreatePlanPrompt.handlebars

@@ -6,4 +6,4 @@
 {{> AdditionalContext }}
 {{> RetryLogic }}
 
-{{> TipsAndInstructions }}
+{{> TipsAndInstructions }}

dotnet/src/Planners/Planners.Handlebars/Handlebars/CreatePlanPromptPartials/TipsAndInstructions.handlebars

@@ -20,7 +20,7 @@ Follow these steps to create one Handlebars template to achieve the goal:
   - Only use the helpers provided. Any helper not listed is considered hallucinated and must not be used.
   - Do not invent or assume the existence of any functions not explicitly defined above.
 3. What if I Need More Helpers?
-  - If the goal cannot be fully achieved with the provided helpers or you need a helper not defined, print the following message: "{{insufficientFunctionsErrorMessage}}".
+  - Stop here if the goal cannot be fully achieved with the provided helpers or you need a helper not defined, and just return a string with an appropriate error message.
 4. Keep It Simple:{{#if allowLoops}}
   - Avoid using loops or block expressions. They are allowed but not always necessary, so try to find a solution that does not use them.{{/if}}
   - Your template should be intelligent and efficient, avoiding unnecessary complexity or redundant steps.

dotnet/src/Planners/Planners.Handlebars/Handlebars/HandlebarsPlanner.cs

@@ -72,11 +72,6 @@ public Task<HandlebarsPlan> CreatePlanAsync(Kernel kernel, string goal, KernelAr
 
     private readonly HandlebarsPromptTemplateFactory _templateFactory;
 
-    /// <summary>
-    /// Error message if kernel does not contain sufficient functions to create a plan.
-    /// </summary>
-    private const string InsufficientFunctionsError = "Additional helpers or information may be required";
-
     private async Task<HandlebarsPlan> CreatePlanCoreAsync(Kernel kernel, string goal, KernelArguments? arguments, CancellationToken cancellationToken = default)
     {
         // Get CreatePlan prompt template
@@ -89,17 +84,10 @@ private async Task<HandlebarsPlan> CreatePlanCoreAsync(Kernel kernel, string goa
         var chatCompletionService = kernel.GetRequiredService<IChatCompletionService>();
         var completionResults = await chatCompletionService.GetChatMessageContentAsync(chatMessages, executionSettings: this._options.ExecutionSettings, cancellationToken: cancellationToken).ConfigureAwait(false);
 
-        // Check if plan could not be created due to insufficient functions
-        if (completionResults.Content is not null && completionResults.Content.IndexOf(InsufficientFunctionsError, StringComparison.OrdinalIgnoreCase) >= 0)
-        {
-            var functionNames = availableFunctions.ToList().Select(func => $"{func.PluginName}{this._templateFactory.NameDelimiter}{func.Name}");
-            throw new KernelException($"[{HandlebarsPlannerErrorCodes.InsufficientFunctionsForGoal}] Unable to create plan for goal with available functions.\nGoal: {goal}\nAvailable Functions: {string.Join(", ", functionNames)}\nPlanner output:\n{completionResults}");
-        }
-
         Match match = Regex.Match(completionResults.Content, @"```\s*(handlebars)?\s*(.*)\s*```", RegexOptions.Singleline);
         if (!match.Success)
         {
-            throw new KernelException($"[{HandlebarsPlannerErrorCodes.InvalidTemplate}] Could not find the plan in the results\nPlanner output:\n{completionResults}");
+            throw new KernelException($"[{HandlebarsPlannerErrorCodes.InvalidTemplate}] Could not find the plan in the results. Additional helpers or input may be required.\n\nPlanner output:\n{completionResults}");
         }
 
         var planTemplate = match.Groups[2].Value.Trim();
@@ -236,7 +224,6 @@ private ChatHistory GetChatHistoryFromPrompt(string prompt)
                 { "goal", goal },
                 { "predefinedArguments", predefinedArgumentsWithTypes},
                 { "nameDelimiter", this._templateFactory.NameDelimiter},
-                { "insufficientFunctionsErrorMessage", InsufficientFunctionsError},
                 { "allowLoops", this._options.AllowLoops },
                 { "complexTypeDefinitions", complexParameterTypes.Count > 0 && complexParameterTypes.Any(p => p.IsComplex) ? complexParameterTypes.Where(p => p.IsComplex) : null},
                 { "complexSchemaDefinitions", complexParameterSchemas.Count > 0 ? complexParameterSchemas : null},