Ship Microsoft.Testing.Platform.Internal.DotnetTest as a preview package (#9400)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

src/Platform/Microsoft.Testing.Platform.Internal.DotnetTest/Microsoft.Testing.Platform.Internal.DotnetTest.csproj

@@ -41,11 +41,20 @@
     <DevelopmentDependency>true</DevelopmentDependency>
 
     <!--
-      IsShipping=false keeps this package OUT of nuget.org and in the internal (NonShipping) feed used for cross-repo
-      dependency flow (dotnet/sdk). NOTE: the exact internal-feed shipping mechanism still needs confirmation with
-      the Arcade team (see PR discussion).
+      IsShipping=true publishes this package (signed) on the same train as the rest of Microsoft.Testing.Platform so
+      it can flow to dotnet/sdk via the normal dependency flow. The 'Internal' in the name plus the always-preview
+      version (see SuppressFinalPackageVersion below) make clear it is an implementation detail, NOT a public API.
     -->
-    <IsShipping>false</IsShipping>
+    <IsShipping>true</IsShipping>
+
+    <!--
+      Keep this package preview even in stabilized/official builds. It inherits VersionPrefix from
+      src/Platform/Directory.Build.props ($(TestingPlatformVersionPrefix)) and the repo-wide PreReleaseVersionLabel (eng/Versions.props),
+      so it ships as <TestingPlatformVersionPrefix>-preview.* today. Without this, a stabilized MTP release
+      (DotNetFinalVersionKind=release) would drop the suffix and publish a STABLE 'Internal' package - which we do not
+      want while the dotnet test shared-source surface is still partial. Mirrors MSTest.SourceGeneration.
+    -->
+    <SuppressFinalPackageVersion>true</SuppressFinalPackageVersion>
 
     <!-- NU5128: source-only package has no lib/ assembly, only contentFiles. -->
     <NoWarn>$(NoWarn);NU5128</NoWarn>

src/Platform/Microsoft.Testing.Platform.Internal.DotnetTest/PACKAGE.md

@@ -7,8 +7,9 @@ stay a **single source of truth** across the [microsoft/testfx](https://github.c
 break).
 
 > ⚠️ This package is an **implementation detail** of `dotnet test` integration — that is what the `Internal` in the
-> name signals. It is `IsShipping=false` (not published to nuget.org) and is **not** intended for direct end-user
-> consumption. (A *public* MTP client/orchestration library was proposed in
+> name signals. It ships as a **preview** package (`IsShipping=true`, always preview via `SuppressFinalPackageVersion`)
+> on the same train as the rest of Microsoft.Testing.Platform so it can flow to dotnet/sdk, and is **not** intended for
+> direct end-user consumption. (A *public* MTP client/orchestration library was proposed in
 > [#5667](https://github.com/microsoft/testfx/issues/5667); that is a separate effort, closed pending a real use
 > case.)