[SequentialPlanner] Add default values to function manual (microsoft#647

)

### Motivation and Context
SequentialPlanner was not respecting default values for inputs.

### Description
This commit modifies the ToManualString extension method for
FunctionView to include the default value of each parameter in the
generated string. This makes the manual more informative and helpful for
LLM to know the default behavior of a function. The default value is
shown in parentheses after the parameter description.

dotnet/src/IntegrationTests/Planning/SequentialPlannerTests.cs

@@ -43,6 +43,7 @@ public async Task CreatePlanFunctionFlowAsync(string prompt, string expectedFunc
 
         // Act
         var plan = await planner.CreatePlanAsync(prompt);
+
         // Assert
         Assert.Contains(
             plan.Steps,
@@ -51,6 +52,28 @@ public async Task CreatePlanFunctionFlowAsync(string prompt, string expectedFunc
                 step.SkillName.Equals(expectedSkill, StringComparison.OrdinalIgnoreCase));
     }
 
+    [Theory]
+    [InlineData("Write a novel about software development that is 3 chapters long.", "NovelOutline", "WriterSkill", "<!--===ENDPART===-->")]
+    public async Task CreatePlanWithDefaultsAsync(string prompt, string expectedFunction, string expectedSkill, string expectedDefault)
+    {
+        // Arrange
+        IKernel kernel = this.InitializeKernel();
+        TestHelpers.GetSkill("WriterSkill", kernel);
+
+        var planner = new SequentialPlanner(kernel);
+
+        // Act
+        var plan = await planner.CreatePlanAsync(prompt);
+
+        // Assert
+        Assert.Contains(
+            plan.Steps,
+            step =>
+                    step.Name.Equals(expectedFunction, StringComparison.OrdinalIgnoreCase) &&
+                    step.SkillName.Equals(expectedSkill, StringComparison.OrdinalIgnoreCase) &&
+                    step.NamedParameters["endMarker"].Equals(expectedDefault, StringComparison.OrdinalIgnoreCase));
+    }
+
     [Theory]
     [InlineData("Write a poem or joke and send it in an e-mail to Kai.", "SendEmailAsync", "_GLOBAL_FUNCTIONS_")]
     public async Task CreatePlanGoalRelevantAsync(string prompt, string expectedFunction, string expectedSkill)

dotnet/src/SemanticKernel/Planning/FunctionViewExtensions.cs

@@ -16,7 +16,7 @@ internal static class FunctionViewExtensions
     /// <returns>A manual-friendly string for a function.</returns>
     internal static string ToManualString(this FunctionView function)
     {
-        var inputs = string.Join("\n", function.Parameters.Select(p => $"    - {p.Name}: {p.Description}"));
+        var inputs = string.Join("\n", function.Parameters.Select(p => $"    - {p.Name}: {p.Description} (default value: {p.DefaultValue}))"));
         return $"  {function.ToFullyQualifiedName()}:\n    description: {function.Description}\n    inputs:\n{inputs}";
     }