Document TestingPlatformBuilderHook auto-registration for MTP extension authors

[Evangelink]

Addresses microsoft/testfx#5552 by documenting the TestingPlatformBuilderHook auto-registration mechanism for Microsoft.Testing.Platform (MTP) extension authors.

What changed

Adds a new ## Auto-register your extension with TestingPlatformBuilderHook section to docs/core/testing/microsoft-testing-platform-architecture-extensions.md that explains:

• Create the hook class – the required public static class TestingPlatformBuilderHook with the AddExtensions(ITestApplicationBuilder, string[]) signature, including a sample mirroring how in-box extensions like Microsoft.Testing.Extensions.Retry ship.
• Declare the MSBuild item – the buildMultiTargeting/<PackageId>.props file that declares a <TestingPlatformBuilderHook> item with Include, DisplayName, and TypeFullName metadata.
• The Include GUID is a random identifier – an explicit subsection clarifying that this GUID is not the same as IExtension.Uid, that it must be freshly generated for every new extension, and (with an [!IMPORTANT] callout) that copying a GUID from another extension causes the hook to be deduplicated away and silently fail to register. Includes a few ways to generate one (PowerShell, Visual Studio, uuidgen).
• Verify the hook is wired up – guidance to inspect the generated SelfRegisteredExtensions.g.cs file and common reasons the call might be missing.

Also adds:

• A [!TIP] callout at the top of the ## Extensibility points section pointing readers to the new section so they don't miss it while reading individual extension docs.
• A ms.date bump.

Why

Issue microsoft/testfx#5552 (from @Youssef1313) called out that the article doesn't tell extension authors how to plug into the auto-generated entry point and doesn't make it clear that the registration GUID must be freshly generated. The article previously only showed the manual builder.TestHost.AddXxx(...) registration calls, leaving authors to reverse-engineer the pattern from in-box extension sources.

Content sources

Section	Source
Hook class shape & signature	Verified against Microsoft.Testing.Extensions.Retry/TestingPlatformBuilderHook.cs and the _expectedItemSpec constant in TestingPlatformAutoRegisteredExtensions.cs.
Props file shape & metadata	Verified against Microsoft.Testing.Extensions.Retry.props and the MSBuild task's metadata constants.
Dedup-by-Include behavior	Verified against ValidateAndDeduplicateBuilderHooks in TestingPlatformAutoRegisteredExtensions.cs.
Generated entry point shape	Verified against GetSourceCode in TestingPlatformAutoRegisteredExtensions.cs.
Consumer opt-out (GenerateTestingPlatformEntryPoint=false)	Consistent with existing wording in microsoft-testing-platform-intro.md, microsoft-testing-platform-features.md, and several extension articles in this repo.

Note

Per repo convention, the AI-usage frontmatter is unchanged (ai-assisted) because this article was already marked as such.

Validation

• npx -y markdownlint-cli2 docs/core/testing/microsoft-testing-platform-architecture-extensions.md → 0 errors.
• Internal anchors used (#auto-register-your-extension-with-testingplatformbuilderhook, #the-include-guid-is-a-random-identifier, ./microsoft-testing-platform-architecture.md#the-iextension-interface) were derived from the corresponding heading text per Docs anchor rules.

Closes microsoft/testfx#5552 (docs side).

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/microsoft-testing-platform-architecture-extensions.md	Build extensions for Microsoft.Testing.Platform (MTP)

[Copilot]

Pull request overview

This pull request updates the Microsoft.Testing.Platform (MTP) extensions architecture documentation to explain how extension authors can participate in MSBuild-driven auto-registration via a TestingPlatformBuilderHook, so consumers don’t need to add manual registration calls in Main.

Changes:

• Adds a new section that documents the TestingPlatformBuilderHook class shape, required method signature, and the MSBuild TestingPlatformBuilderHook item in a buildMultiTargeting/<PackageId>.props file.
• Adds guidance about the Include GUID’s purpose (deduplication) and how to verify wiring via the generated SelfRegisteredExtensions.g.cs.
• Adds a tip callout near the top of Extensibility points pointing readers to the new auto-registration section and bumps ms.date.

Show a summary per file

File	Description
docs/core/testing/microsoft-testing-platform-architecture-extensions.md	Documents TestingPlatformBuilderHook auto-registration, adds a cross-reference tip, and updates the article date.

Copilot's findings

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• Comments generated: 1