Fix mock authentication in ASP.NET Core integration tests docs

[Copilot]

• Understand the issue: AddAuthentication(defaultScheme: "TestScheme") sets only DefaultScheme (a fallback), but the app's existing authentication configuration sets DefaultAuthenticateScheme and DefaultChallengeScheme explicitly, which take precedence and override the test scheme
• Update snippet3 in xunit AuthTests.cs to explicitly set DefaultAuthenticateScheme and DefaultChallengeScheme
• Update snippet3 in mstest AuthTests.cs to explicitly set DefaultAuthenticateScheme and DefaultChallengeScheme
• Update snippet3 in nunit AuthTests.cs to explicitly set DefaultAuthenticateScheme and DefaultChallengeScheme
• Update highlight line numbers in integration-tests9.md from 7-12 to 7-16 to cover the newly expanded code
• Update the explanatory text in integration-tests9.md to explain why explicit scheme configuration is needed
• Update ms.date in integration-tests.md to today's date (03/10/2026)
• Revert .gitignore change per reviewer request

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Mock Authentication] Integration tests in ASP.NET Core</issue_title>
<issue_description>### Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

https://learn.microsoft.com/en-us/aspnet/core/test/integration-tests?view=aspnetcore-7.0#mock-authentication

This code doesn't work. Authentication fails. You need to introduce additional settings to the authentication service. Change the code from this

[Fact]
public async Task Get_SecurePageIsReturnedForAnAuthenticatedUser()
{
    // Arrange
    var client = _factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureTestServices(services =>
            {
                services.AddAuthentication(defaultScheme: "TestScheme")
                    .AddScheme<AuthenticationSchemeOptions, TestAuthHandler>(
                        "TestScheme", options => { });
            });
        })
        .CreateClient(new WebApplicationFactoryClientOptions
        {
            AllowAutoRedirect = false,
        });

    client.DefaultRequestHeaders.Authorization =
        new AuthenticationHeaderValue(scheme: "TestScheme");

    //Act
    var response = await client.GetAsync("/SecurePage");

    // Assert
    Assert.Equal(HttpStatusCode.OK, response.StatusCode);
}

to this

[Fact]
public async Task Get_SecurePageIsReturnedForAnAuthenticatedUser()
{
    // Arrange
    var client = _factory.WithWebHostBuilder(builder =>
        {
            builder.ConfigureTestServices(services =>
            {
                services.AddAuthentication(o =>
                    {
                        o.DefaultAuthenticateScheme = "TestScheme";
                        o.DefaultChallengeScheme = "TestScheme";
                    }).AddScheme<AuthenticationSchemeOptions, TestAuthHandler>(
                        "TestScheme", options => { });
            });
        })
        .CreateClient(new WebApplicationFactoryClientOptions
        {
            AllowAutoRedirect = false,
        });

    client.DefaultRequestHeaders.Authorization =
        new AuthenticationHeaderValue(scheme: "TestScheme");

    //Act
    var response = await client.GetAsync("/SecurePage");

    // Assert
    Assert.Equal(HttpStatusCode.OK, response.StatusCode);
}

Expected Behavior

You should be able to acces authenticated pages when you've mocked the authentication.

Steps To Reproduce

Implement the "mock authentication" steps and try to access a page that requires authentication. You will receive a 401.

Exceptions (if any)

The HTTPClient returns 401 even when you're mocking the authentication.

.NET Version

7.0.201

Anything else?

https://learn.microsoft.com/en-us/aspnet/core/test/integration-tests?view=aspnetcore-7.0#mock-authentication

Page URL

https://learn.microsoft.com/en-us/aspnet/core/test/integration-tests?view=aspnetcore-8.0#client-options

Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/test/integration-tests.md

Document ID

f499b131-8b14-7e10-1728-543fdd5a5656

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Associated WorkItem - 340922</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Mock Authentication] Integration tests in ASP.NET Core #31717

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[tdykstra]

Looks good!