Document Assert.That custom assertions and de-emphasize StringAssert/CollectionAssert

[Evangelink]

Summary

Updates the MSTest assertions article to:

1. Document the Assert.That, StringAssert.That, and CollectionAssert.That extensibility hook (the singleton property used as a target for user-authored extension methods, per testfx RFC 002).
2. Strengthen the guidance that StringAssert and CollectionAssert are not recommended for new code and are likely to be deprecated. The main reason is discoverability: splitting assertions across three types means you can't see all available checks in a single IntelliSense list.

Contributes to microsoft/testfx#8818

Changes

• New section Create custom assertions with Assert.That with:
  • A C# extension method example (IsPrime) that targets the Assert instance type.
  • A call-site example showing Assert.That.IsPrime(result).
  • A subsection acknowledging StringAssert.That and CollectionAssert.That exist for the same pattern, but recommending Assert.That for new code.
  • A disambiguation NOTE clarifying the singleton property (Assert.That.MyAssertion(...)) is distinct from the MSTest 3.8 expression-tree method (Assert.That(() => condition)).
• Strengthened deprecation messaging on StringAssert and CollectionAssert (NOTE → WARNING), and updated the top-of-doc callout from TIP → IMPORTANT explaining the discoverability problem.
• New best-practice bullet recommending custom assertions extend Assert.That (not StringAssert.That/CollectionAssert.That).
• Added ai-usage: ai-assisted and bumped ms.date.

Verification

• npx markdownlint-cli2 passes with 0 errors.
• All three new <xref:...> API IDs (Assert.That, StringAssert.That, CollectionAssert.That) resolve against the Microsoft Learn API browser.

Content source

• API surface and extensibility pattern: verified against microsoft/testfx sources (Assert.cs, StringAssert.cs, CollectionAssert.cs, Assert.That.cs) and RFC 002 — Framework Extensibility for Custom Assertions.
• "Likely to be deprecated" wording reflects @Evangelink's guidance in the issue prompt; no formal [Obsolete] attribute is shipped yet.

---

Internal previews

📄 File	🔗 Preview link
docs/core/testing/unit-testing-mstest-writing-tests-assertions.md	MSTest assertions

[Copilot]

Pull request overview

Updates the MSTest assertions documentation to explain the *.That extensibility hook for custom assertions and to steer new code away from StringAssert/CollectionAssert due to discoverability and likely future deprecation.

Changes:

• Replaces the top callout with stronger guidance to prefer Assert and warns about likely deprecation of StringAssert/CollectionAssert.
• Adds a new section documenting custom assertions via Assert.That (and mentions StringAssert.That/CollectionAssert.That).
• Updates best-practices guidance to recommend extending Assert.That, and adds ai-usage plus an updated ms.date.

Show a summary per file

File	Description
docs/core/testing/unit-testing-mstest-writing-tests-assertions.md	Adds Assert.That extensibility documentation and strengthens guidance away from StringAssert/CollectionAssert.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 2

[Evangelink]

@meaghanlewis @gewarren could you please review?

[Evangelink]

@gewarren I am not sure how to fix the error on this PR, could you please help?

[gewarren]

@gewarren I am not sure how to fix the error on this PR, could you please help?

They've disabled OPS builds since the weekend. They hope to turn them back on today :-S.