.Net: Invariant culture for OpenApiSchema conversion (#4540)

Closes : #4504

### Motivation, Context and Description
The issue occurred during the conversion from OpenApiSchema to
KernelJsonSchema on environments with a culture that uses a comma as a
decimal separator. To resolve this, the
OpenApiSchemaExtensions.ToJsonSchema method has been updated to utilize
the invariant culture for conversion instead of using current culture.

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raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/Functions/Functions.OpenApi/Extensions/OpenApiSchemaExtensions.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System.Globalization;
 using System.IO;
 using System.Text;
 using Microsoft.OpenApi.Models;
@@ -17,7 +18,7 @@ internal static class OpenApiSchemaExtensions
     internal static KernelJsonSchema ToJsonSchema(this OpenApiSchema schema)
     {
         var schemaBuilder = new StringBuilder();
-        var jsonWriter = new OpenApiJsonWriter(new StringWriter(schemaBuilder));
+        var jsonWriter = new OpenApiJsonWriter(new StringWriter(schemaBuilder, CultureInfo.InvariantCulture));
         jsonWriter.Settings.InlineLocalReferences = true;
         schema.SerializeAsV3(jsonWriter);
         return KernelJsonSchema.Parse(schemaBuilder.ToString());

dotnet/src/Functions/Functions.UnitTests/OpenApi/Extensions/OpenApiSchemaExtensionsTests.cs

@@ -0,0 +1,49 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Globalization;
+using Microsoft.OpenApi.Any;
+using Microsoft.OpenApi.Models;
+using Microsoft.SemanticKernel.Plugins.OpenApi;
+using Xunit;
+
+namespace SemanticKernel.Functions.UnitTests.OpenApi.Extensions;
+public class OpenApiSchemaExtensionsTests
+{
+    [Fact]
+    public void ItShouldConvertOpenApiSchemaUsingInvariantCulture()
+    {
+        // Arrange
+        var schema = new OpenApiSchema
+        {
+            Type = "object",
+            Properties = new Dictionary<string, OpenApiSchema>
+            {
+                ["property1"] = new OpenApiSchema
+                {
+                    Type = "number",
+                    Format = "double",
+                    Default = new OpenApiDouble(12.01)
+                }
+            }
+        };
+
+        var currentCulture = CultureInfo.CurrentCulture; // Backup current culture
+
+        // Act & Assert
+        try
+        {
+            CultureInfo.CurrentCulture = new CultureInfo("fr-FR"); // French culture uses comma as decimal separator
+
+            var result = OpenApiSchemaExtensions.ToJsonSchema(schema); // Should use invariant culture
+
+            Assert.True(result.RootElement.TryGetProperty("properties", out var properties));
+            Assert.True(properties.TryGetProperty("property1", out var property2));
+            Assert.Equal(12.01, property2.GetProperty("default").GetDouble());
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = currentCulture; // Restore current culture
+        }
+    }
+}