.Net: Adding #each block details + jsonSerializer fix to handle named…

… FP literals (#5183)

### Motivation and Context

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Adding #each block details + jsonSerializer fix to handle named FP
literals

Resolves #4838

### 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/samples/KernelSyntaxExamples/Example65_HandlebarsPlanner.cs

@@ -140,18 +140,25 @@ private void PrintPlannerDetails(string goal, HandlebarsPlan plan, string result
     [InlineData(false)]
     public async Task PlanNotPossibleSampleAsync(bool shouldPrintPrompt)
     {
-        WriteSampleHeading("Plan Not Possible");
-
-        // Load additional plugins to enable planner but not enough for the given goal.
-        await RunSampleAsync("Send Mary an email with the list of meetings I have scheduled today.", null, null, shouldPrintPrompt, true, "SummarizePlugin");
-        /*
-            [InsufficientFunctionsForGoal] Unable to create plan for goal with available functions.
-            Goal: Send Mary an email with the list of meetings I have scheduled today.
-            Available Functions: SummarizePlugin-MakeAbstractReadable, SummarizePlugin-Notegen, SummarizePlugin-Summarize, SummarizePlugin-Topics
-            Planner output:
-            As the available helpers do not contain any functionality to send an email or interact with meeting scheduling data, I cannot create a template to achieve the stated goal. 
-            Additional helpers or information may be required.
-        */
+        try
+        {
+            WriteSampleHeading("Plan Not Possible");
+
+            // Load additional plugins to enable planner but not enough for the given goal.
+            await RunSampleAsync("Send Mary an email with the list of meetings I have scheduled today.", null, null, shouldPrintPrompt, true, "SummarizePlugin");
+            /*
+                [InsufficientFunctionsForGoal] Unable to create plan for goal with available functions.
+                Goal: Send Mary an email with the list of meetings I have scheduled today.
+                Available Functions: SummarizePlugin-MakeAbstractReadable, SummarizePlugin-Notegen, SummarizePlugin-Summarize, SummarizePlugin-Topics
+                Planner output:
+                As the available helpers do not contain any functionality to send an email or interact with meeting scheduling data, I cannot create a template to achieve the stated goal. 
+                Additional helpers or information may be required.
+            */
+        }
+        catch (Exception e)
+        {
+            WriteLine(e.InnerException?.Message);
+        }
     }
 
     [RetryTheory(typeof(HttpOperationException))]
@@ -325,11 +332,11 @@ public Task RunPredefinedVariablesSampleAsync(bool shouldPrintPrompt)
         return RunSampleAsync("Write a poem about the given person, then translate it into French.", null, initialArguments, shouldPrintPrompt, true, "WriterPlugin", "MiscPlugin");
         /*
             Original plan:
-            {{!-- Step 0: Set the given person --}}
-            {{set "person" "John Doe"}}
+            {{!-- Step 0: Extract key values --}}
+            {{set "personName" @root.person.name}}
 
             {{!-- Step 1: Generate a short poem about the person --}}
-            {{set "poem" (WriterPlugin-ShortPoem input=person)}}
+            {{set "poem" (WriterPlugin-ShortPoem input=personName)}}
 
             {{!-- Step 2: Translate the poem into French --}}
             {{set "translatedPoem" (WriterPlugin-Translate input=poem language="French")}}

dotnet/src/Extensions/PromptTemplates.Handlebars/Helpers/KernelHelpers/KernelSystemHelpers.cs

@@ -14,6 +14,14 @@ namespace Microsoft.SemanticKernel.PromptTemplates.Handlebars.Helpers;
 /// </summary>
 internal static class KernelSystemHelpers
 {
+    /// <summary>
+    /// The "NaN", "Infinity", and "-Infinity" String tokens can be read as floating-point constants, and the Single and Double values for these constants will be written as their corresponding JSON string representations.
+    /// </summary>
+    private static readonly JsonSerializerOptions s_jsonSerializerOptions = new()
+    {
+        NumberHandling = System.Text.Json.Serialization.JsonNumberHandling.AllowNamedFloatingPointLiterals
+    };
+
     /// <summary>
     /// Register all (default) or specific categories of system helpers.
     /// </summary>
@@ -91,11 +99,13 @@ internal static class KernelSystemHelpers
             var args = ProcessArguments(arguments, variables);
             object objectToSerialize = args[0];
 
-            return objectToSerialize switch
+            object v = objectToSerialize switch
             {
                 string stringObject => objectToSerialize,
-                _ => JsonSerializer.Serialize(objectToSerialize)
+                _ => JsonSerializer.Serialize(objectToSerialize, s_jsonSerializerOptions)
             };
+
+            return v;
         });
 
         handlebarsInstance.RegisterHelper("concat", (in HelperOptions options, in Context context, in Arguments arguments) =>

dotnet/src/Planners/Planners.Handlebars/Handlebars/CreatePlanPromptPartials/Helpers/BlockHelpers.handlebars

@@ -3,6 +3,9 @@
 ## Built-in block helpers
 - `\{{#if}}\{{/if}}`
 - `\{{#unless}}\{{/unless}}`{{#if allowLoops}}
-- `\{{#each}}\{{/each}}`{{/if}}
+- `\{{#each}}\{{/each}}` - inside this block, you can use:
+    - `this` to reference the element being iterated over
+    - `@index` to reference the current index
+    - `@key` to reference the current key (for object iteration){{/if}}
 - `\{{#with}}\{{/with}}`
 {{/inline}}