[release/11.0.1xx-preview7] Support Microsoft.Build.Traversal projects in dotnet test (MTP)

[Evangelink]

Backport of #55297 to release/11.0.1xx-preview7.

Summary

• support Microsoft.Build.Traversal projects in dotnet test with Microsoft.Testing.Platform
• expand nested traversal graphs while de-duplicating diamond references
• forward per-reference Configuration and Platform metadata during build and evaluation
• add direct and nested traversal integration coverage

Validation

• build.cmd -c Debug
• dotnet.Tests incremental build
• traversal MTP tests: 4 passed (Debug and Release for both direct and nested/diamond scenarios)

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[Copilot]

Pull request overview

Adds Microsoft.Build.Traversal (dirs.proj) support to dotnet test when using Microsoft.Testing.Platform (MTP), by expanding traversal graphs into referenced projects (including nested/diamond graphs), ensuring referenced test projects are built, and validating behavior with new integration assets/tests.

Changes:

• Teach MTP dotnet test project evaluation to detect traversal projects and recursively expand ProjectReference items with cross-graph de-duplication and per-reference Configuration/Platform forwarding.
• Treat any *proj extension as a positional project argument so .proj inputs aren’t forwarded to the test host as invalid arguments.
• Update _MTPBuild to forward the build to traversal @(ProjectReference) items and add direct + nested/diamond traversal integration coverage.

Show a summary per file

File	Description
test/TestAssets/TestProjects/TraversalTestProjects/dirs.proj	New traversal asset (direct) listing referenced MTP test projects.
test/TestAssets/TestProjects/TraversalTestProjects/global.json	Selects MTP runner for the traversal asset.
test/TestAssets/TestProjects/TraversalTestProjects/TestProject/TestProject.csproj	New MTP test app project referenced by traversal.
test/TestAssets/TestProjects/TraversalTestProjects/TestProject/Program.cs	Dummy MTP adapter app entry point.
test/TestAssets/TestProjects/TraversalTestProjects/OtherTestProject/OtherTestProject.csproj	Second new MTP test app project referenced by traversal.
test/TestAssets/TestProjects/TraversalTestProjects/OtherTestProject/Program.cs	Dummy MTP adapter app entry point.
test/TestAssets/TestProjects/TraversalTestProjectsNested/dirs.proj	New traversal asset (nested/diamond) for recursion + de-dup testing.
test/TestAssets/TestProjects/TraversalTestProjectsNested/sub/dirs.proj	Nested traversal project forming the diamond graph.
test/TestAssets/TestProjects/TraversalTestProjectsNested/global.json	Selects MTP runner for the nested traversal asset.
test/TestAssets/TestProjects/TraversalTestProjectsNested/SharedTestProject/SharedTestProject.csproj	Shared MTP test app referenced by multiple traversal nodes.
test/TestAssets/TestProjects/TraversalTestProjectsNested/SharedTestProject/Program.cs	Writes marker files to deterministically assert de-dup behavior.
test/TestAssets/TestProjects/TraversalTestProjectsNested/LeafTestProject/LeafTestProject.csproj	Leaf MTP test app reachable only via nested traversal.
test/TestAssets/TestProjects/TraversalTestProjectsNested/LeafTestProject/Program.cs	Writes marker files to deterministically assert recursion behavior.
test/dotnet.Tests/CommandTests/Test/GivenDotnetTestBuildsAndRunsTests.cs	Adds direct + nested/diamond traversal integration tests.
src/Cli/dotnet/Commands/Test/CliConstants.cs	Adds constants for traversal detection and ProjectReference item access.
src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs	Recognizes any *proj positional project argument (including .proj).
src/Cli/dotnet/Commands/Test/MTP/SolutionAndProjectUtility.cs	Adds traversal-project expansion logic with de-dup + config/platform forwarding.
src/Layout/redist/MSBuildImports/Current/Microsoft.Common.targets/ImportAfter/Microsoft.TestPlatform.ImportAfter.targets	Forwards _MTPBuild through traversal project references and forwards per-reference config/platform.

Copilot's findings

• Files reviewed: 18/18 changed files
• Comments generated: 3