Add AssumeCultureAttribute for locale-specific testing and update API descriptions

The AssumeCultureAttribute is, for now, copied from another test project. PR feedback can indicate if a shared approach is preferred, and, if so, how that can be achieved

Author: sander1095

src/AspNetCore/WebApi/test/Asp.Versioning.OpenApi.Tests/AssumeCultureAttribute.cs

@@ -0,0 +1,39 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+
+namespace Asp.Versioning;
+
+using System.Globalization;
+using System.Reflection;
+using static System.AttributeTargets;
+using static System.Threading.Thread;
+
+/// <summary>
+/// Allows a test method to assume that it is running in a specific locale.
+/// </summary>
+[AttributeUsage( Class | Method, AllowMultiple = false, Inherited = true )]
+internal sealed class AssumeCultureAttribute : BeforeAfterTestAttribute
+{
+    private CultureInfo originalCulture;
+    private CultureInfo originalUICulture;
+
+    public AssumeCultureAttribute( string name ) => Name = name;
+
+    public string Name { get; }
+
+    public override void Before( MethodInfo methodUnderTest, IXunitTest test )
+    {
+        originalCulture = CurrentThread.CurrentCulture;
+        originalUICulture = CurrentThread.CurrentUICulture;
+
+        var culture = CultureInfo.CreateSpecificCulture( Name );
+
+        CurrentThread.CurrentCulture = culture;
+        CurrentThread.CurrentUICulture = culture;
+    }
+
+    public override void After( MethodInfo methodUnderTest, IXunitTest test )
+    {
+        CurrentThread.CurrentCulture = originalCulture;
+        CurrentThread.CurrentUICulture = originalUICulture;
+    }
+}

src/AspNetCore/WebApi/test/Asp.Versioning.OpenApi.Tests/Content/v1-minimal.json

@@ -2,7 +2,7 @@
   "openapi": "3.1.1",
   "info": {
     "title": "Test API | v1",
-    "description": "The API was deprecated on 01/01/2026. The API was sunset on 02/10/2026.\n\n### Links\n\n- [Version Deprecation Policy](http://my.api.com/policies/versions/deprecated.html)\n- [Version Sunset Policy](http://my.api.com/policies/versions/sunset.html)",
+    "description": "The API was deprecated on 1/1/2026. The API was sunset on 2/10/2026.\r\n\r\n### Links\r\n\r\n- [Version Deprecation Policy](http://my.api.com/policies/versions/deprecated.html)\r\n- [Version Sunset Policy](http://my.api.com/policies/versions/sunset.html)",
     "version": "1.0"
   },
   "servers": [

src/AspNetCore/WebApi/test/Asp.Versioning.OpenApi.Tests/Content/v1-mixed.json

@@ -2,7 +2,7 @@
   "openapi": "3.1.1",
   "info": {
     "title": "Test API | v1",
-    "description": "The API was deprecated on 01/01/2026. The API was sunset on 02/10/2026.\n\n### Links\n\n- [Version Deprecation Policy](http://my.api.com/policies/versions/deprecated.html)\n- [Version Sunset Policy](http://my.api.com/policies/versions/sunset.html)",
+    "description": "The API was deprecated on 1/1/2026. The API was sunset on 2/10/2026.\r\n\r\n### Links\r\n\r\n- [Version Deprecation Policy](http://my.api.com/policies/versions/deprecated.html)\r\n- [Version Sunset Policy](http://my.api.com/policies/versions/sunset.html)",
     "version": "1.0"
   },
   "servers": [

src/AspNetCore/WebApi/test/Asp.Versioning.OpenApi.Tests/Transformers/AcceptanceTest.cs

@@ -7,6 +7,7 @@ namespace Asp.Versioning.OpenApi.Transformers;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.DependencyInjection;
+using System.Globalization;
 using System.Net.Http.Json;
 using System.Text.Json.Nodes;
 
@@ -20,11 +21,12 @@ public class AcceptanceTest
     /// </summary>
     /// <returns>A <see cref="Task"/> representing the asynchronous unit test.</returns>
     [Fact]
+    [AssumeCulture( "en-US" )]
     public async Task minimal_api_should_generate_expected_open_api_document()
     {
         // arrange
         var builder = WebApplication.CreateBuilder();
-
+        var culture = CultureInfo.CurrentCulture;
         builder.WebHost.UseTestServer();
         builder.Services.AddApiVersioning( options => AddPolicies( options ) )
                         .AddApiExplorer( options => options.GroupNameFormat = "'v'VVV" )
@@ -57,6 +59,7 @@ public async Task minimal_api_should_generate_expected_open_api_document()
     }
 
     [Fact]
+    [AssumeCulture( "en-US" )]
     public async Task controller_should_generate_expected_open_api_document()
     {
         // arrange
@@ -91,6 +94,7 @@ public async Task controller_should_generate_expected_open_api_document()
     }
 
     [Fact]
+    [AssumeCulture( "en-US" )]
     public async Task mixed_api_should_generate_expected_open_api_document()
     {
         // arrange