Document Assert.Scope() soft assertions and related behaviors

[Evangelink]

Summary

Documents the experimental Assert.Scope() soft-assertions API and its behaviors in the MSTest assertions article. This API previously had no coverage in dotnet/docs.

Added a new Soft assertions with Assert.Scope() section under The Assert class, covering:

• What soft assertions are and how failures are collected and reported together on Dispose (single failure vs. AggregateException for multiple).
• Experimental MSTESTEXP diagnostic note.
• Postconditions are not enforced inside a scope (nullability/type narrowing); downstream exceptions are secondary symptoms of the already-collected failure.
• Value-returning assertions (Assert.Throws*, Assert.ThrowsExactly*, Assert.ContainsSingle) return null/default on failure inside a scope — don't rely on the return value. (Addresses confusion seen in Assert.Throws<T>() / Assert.ThrowsExactly<T>() returns null when assertion fails inside AssertScope, violating non-nullable return contract microsoft/testfx#9069.)
• Assert.Fail / Assert.Inconclusive always throw, even inside a scope.
• Nested scopes are not supported.

This mirrors the design captured in MSTest RFC 011 (Soft Assertions and Nullability Design).

Related

• Assert.Throws<T>() / Assert.ThrowsExactly<T>() returns null when assertion fails inside AssertScope, violating non-nullable return contract microsoft/testfx#9069

---

Internal previews

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

[Copilot]

Pull request overview

This PR documents the experimental Assert.Scope() soft-assertions API in the MSTest assertions article, explaining how failures are collected and reported at scope disposal, and calling out key behavioral differences from normal assertions.

Changes:

• Added a new “Soft assertions with Assert.Scope()” section under The Assert class.
• Documented disposal-time failure reporting behavior, postcondition limitations, and value-returning assertion behavior inside a scope.
• Updated the article’s ms.date.

Show a summary per file

File	Description
docs/core/testing/unit-testing-mstest-writing-tests-assertions.md	Adds documentation for Assert.Scope() soft assertions, including experimental-status guidance and behavioral caveats.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 3

[Evangelink]

@copilot resolve the merge conflicts in this pull request

[Evangelink]

Ping @gewarren