Exclude non-equivalent async alternatives from VSTHRD103

[Andrew Arnott (AArnott)]

VSTHRD103 assumes an Async-suffixed counterpart is generally preferable, but several framework APIs use the async variant for different semantics or specialized cases. This causes incorrect recommendations for synchronous assertions and Entity Framework Core change tracking APIs.

Changes

• Ship package-provided AdditionalFile exclusions for xUnit Throws/ThrowsAny, NUnit Assert.That, and EF Core DbContext/DbSet<TEntity> Add/AddRange.
• Keep the policy data-driven so projects and NuGet packages can extend the same exclusion file pattern for their own APIs.
• Document the built-in exclusions and add regression coverage that confirms matching remains scoped to exact containing types.

The task-flow case in #1662 is intentionally not addressed here because it is being fixed separately through semantic analysis.

Validation

• All 98 VSTHRD103 analyzer tests pass on net8.0.
• The packed analyzer includes the exclusion file under buildTransitive/AdditionalFiles.

Fixes #1661
Fixes #1480

[Copilot]

Pull request overview

This PR refines VSTHRD103 (“Use async methods when in an async method”) to avoid recommending Async-suffixed alternatives that are known to be non-equivalent for several common framework APIs (notably xUnit assertions, NUnit assertions, and EF Core change tracking APIs). It does this via a package-shipped AdditionalFiles exclusion list while keeping the approach extensible through the existing filename-pattern mechanism.

Changes:

• Add a buildTransitive AdditionalFiles exclusion list (vs-threading.SyncMethodsToExcludeFromVSTHRD103.frameworks.txt) covering xUnit, NUnit, and EF Core methods with non-equivalent async counterparts.
• Add a regression test to ensure the known framework exclusions suppress diagnostics while similarly-named methods on other types still produce diagnostics.
• Update analyzer documentation to describe the package-provided default exclusions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD103UseAsyncOptionAnalyzerTests.cs	Adds regression coverage for the new built-in exclusion cases and verifies type scoping.
src/Microsoft.VisualStudio.Threading.Analyzers.CodeFixes/buildTransitive/AdditionalFiles/vs-threading.SyncMethodsToExcludeFromVSTHRD103.frameworks.txt	Introduces the package-shipped default exclusion list for VSTHRD103.
docfx/analyzers/VSTHRD103.md	Minor documentation formatting tweak in the analyzer page.
docfx/analyzers/configuration.md	Documents the built-in exclusions and how to extend exclusions via AdditionalFiles.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.