[PROTOTYPE] Add Microsoft.NET.Sdk.Testing skeleton (#49294)

[Evangelink]

Warning

Draft / prototype, not for merge. Opening this so the shape of #49294 can be discussed against concrete code rather than just a design doc. The MTP path for xUnit/NUnit/TUnit is the only one anywhere close to validated; MSTest support is naive (a real impl should delegate to MSTest.Sdk); no template integration; no test asset yet; strings are inline (no .resx/.xlf yet).

What this is

Prototype of Microsoft.NET.Sdk.Testing, the proposal in #49294, as an in-tree MSBuild SDK shipped in-band with the .NET SDK. Layout mirrors src/WebSdk/Worker/ so MSBuild''s SDK resolver picks it up the same way the existing per-area SDKs (Microsoft.NET.Sdk.Worker, Microsoft.NET.Sdk.Razor, etc.) are.

Design decisions reflected in this prototype

1. Framework + platform are two properties, platform defaults from framework. Valid combos enforced at evaluation:

   Framework	MTP	VSTest	Default when omitted
   MSTest	✅	✅	MTP
   NUnit	✅	✅	MTP
   XUnit (v3)	✅	✅	MTP
   TUnit	✅	❌	MTP
   Expecto	✅	✅	MTP

2. Frameworks come from NuGet — nothing framework-specific is shipped in-band. Avoids the maintenance/CVE-response burden of in-banding code the SDK team doesn''t own. Every <Framework>Version (and companion package version) has a pinned default in Targets/_DefaultPackageVersions.props so projects don''t need to specify a version. Users can override with a fixed version (recommended for reproducibility) or with * / *-* to opt into NuGet floating. The defaults shipped here are placeholders for the prototype; in the real SDK they would be sourced from eng/Versions.props at build time so the matrix advances on each .NET SDK release.

3. MTP extensions surface mirrors MSTest.Sdk (TestingExtensionsProfile + per-extension Enable<Name> and <Name>Version), applied uniformly across frameworks because the extensions hook MTP, not the framework.

4. <Framework>PackageSource local-path override (Arcade-style) is not in this prototype; deferred to v2 per the design discussion.

5. IsTestProject semantic conflict is handled additively: IsTestProject stays VSTest-owned (set by Microsoft.NET.Test.Sdk package), IsTestUtilityProject is the new evaluation-time signal. Zero regression for _CalculateIsVSTestTestProject, IsRidAgnostic, and Arcade.

Layout

src/TestingSdk/
├── README.md
├── Sdk/
│   ├── Sdk.props                                    # MSBuild SDK entry
│   └── Sdk.targets
├── Targets/
│   ├── Microsoft.NET.Sdk.Testing.props              # Dispatch + validation
│   ├── Microsoft.NET.Sdk.Testing.targets
│   ├── Frameworks/
│   │   ├── MSTest.{props,targets}
│   │   ├── NUnit.{props,targets}
│   │   ├── XUnit.{props,targets}
│   │   ├── TUnit.{props,targets}
│   │   └── Expecto.{props,targets}
│   └── Platforms/
│       ├── MicrosoftTestingPlatform.{props,targets}
│       └── VSTest.{props,targets}
└── Tasks/
    └── Microsoft.NET.Sdk.Testing.Tasks.csproj       # Packaging only

Plus wiring in:

• src/Layout/redist/targets/GenerateLayout.targets — PublishNetSdks builds the new csproj and lays out Sdks/Microsoft.NET.Sdk.Testing/.
• sdk.slnx, source-build.slnf.

Sample consuming project

<Project Sdk="Microsoft.NET.Sdk.Testing">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <TestFramework>XUnit</TestFramework>
    <TestingExtensionsProfile>AllMicrosoft</TestingExtensionsProfile>
  </PropertyGroup>
</Project>

The xUnit version is omitted above — the SDK supplies a pinned default. Add <XUnitVersion>3.0.0</XUnitVersion> to lock to a specific version, or <XUnitVersion>*</XUnitVersion> to float to the latest stable on each restore.

What I''d like reviewers to push back on

1. MSTest carve-out: delegate to MSTest.Sdk (recommended) vs re-implement inline (current prototype)?
2. Token spelling: MicrosoftTestingPlatform (canonical) + MTP (alias) — fine, or pick one?
3. XUnit = always v3 with no v2 path — confirmed direction?
4. Default versions vs * floating vs explicit pins — the prototype now ships pinned defaults and exposes all three modes (no property = SDK default, <Version>X.Y</Version> = explicit pin, <Version>*</Version> = float). Reasonable tradeoff?
5. Validating with BeforeTargets="CollectPackageReferences;BeforeBuild" for required-property checks — early enough? Better target?

Known gaps (also in src/TestingSdk/README.md)

• MSTest path is inline; should delegate to MSTest.Sdk.
• No dotnet new template changes (lives in dotnet/templating).
• No Features/Aspire.targets, Features/Playwright.targets, Runner/NativeAOT.targets analogues yet.
• No <Framework>PackageSource (v2).
• No test asset under test/TestAssets/TestProjects yet.
• Error messages inline; should move to .resx/.xlf for localization.
• Has not yet been validated end-to-end through a build.cmd + dogfood + dotnet new + dotnet test cycle on my machine — CI on this PR will be the first real exercise.

Closes: nothing — keep #49294 open for design discussion.