.Net: Fix #10389 (#10406)

### Motivation and Context

Fixes issue #10389.

### Description

Use `JsonSerializerOptions.Encoder =
JavaScriptEncoder.UnsafeRelaxedJsonEscaping` to generate more LLM
friendly serialized FunctionResult.

### 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 😄

---------

Co-authored-by: SergeyMenshykh <68852919+SergeyMenshykh@users.noreply.github.com>

Author: RamType0

dotnet/src/InternalUtilities/connectors/AI/FunctionCalling/FunctionCallsProcessor.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using System.Text.Encodings.Web;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
@@ -490,6 +491,17 @@ public static string ProcessFunctionResult(object functionResult)
             return chatMessageContent.ToString();
         }
 
-        return JsonSerializer.Serialize(functionResult);
+        return JsonSerializer.Serialize(functionResult, s_functionResultSerializerOptions);
     }
+
+    /// <summary>
+    /// The <see cref="JsonSerializerOptions" /> which will be used in <see cref="ProcessFunctionResult(object)"/>.
+    /// </summary>
+    /// <remarks>
+    /// <see cref="JsonSerializer.Serialize{TValue}(TValue, JsonSerializerOptions?)"/> is very likely to escape characters and generates LLM unfriendly results by default.
+    /// </remarks>
+    private static readonly JsonSerializerOptions s_functionResultSerializerOptions = new()
+    {
+        Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
+    };
 }

dotnet/src/SemanticKernel.UnitTests/Utilities/AIConnectors/FunctionCallsProcessorTests.cs

@@ -824,6 +824,19 @@ public void ItShouldSerializeFunctionResultsOfComplexType()
         Assert.Equal("{\"a\":2,\"b\":\"test\"}", result);
     }
 
+    [Fact]
+    public void ItShouldSerializeFunctionResultsWithStringProperties()
+    {
+        // Arrange
+        var functionResult = new { Text = "ﾃｽﾄ" };
+
+        // Act
+        var result = FunctionCallsProcessor.ProcessFunctionResult(functionResult);
+
+        // Assert
+        Assert.Equal("{\"Text\":\"ﾃｽﾄ\"}", result);
+    }
+
     private sealed class AutoFunctionInvocationFilter(
         Func<AutoFunctionInvocationContext, Func<AutoFunctionInvocationContext, Task>, Task>? onAutoFunctionInvocation) : IAutoFunctionInvocationFilter
     {