Add test project for Microsoft.TestPlatform.Filter.Source source-only package

[Copilot]

Adds a test project that validates the Microsoft.TestPlatform.Filter.Source source-only package by compiling its source files directly (as end-users would via NuGet), without referencing the csproj.

Key design points

• No project reference — source files are pulled in via <Compile Include="..." /> items, mirroring real consumer usage
• IsFilterSourcePackage=true — prevents IS_VSTEST_REPO from being defined, so the internal sealed/[Embedded] code paths are exercised
• Conflict isolation — a BeforeTargets="ResolveProjectReferences" target removes the FromP2P vstest project references added by Directory.Build.targets, which would otherwise cause CS0436 type conflicts between the inlined source and the compiled vstest assemblies

Tests covered

• FilterHelper — Escape, Unescape, invalid escape sequences, roundtrip
• FilterExpressionWrapper — parse error detection, Evaluate with =, !=, ~, &, |, ValidForProperties, fast-filter construction
• TestCaseFilterExpression — MatchTestCase, ValidForProperties, null guard on constructor, parse-error short-circuit

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• pkvvsblobprodcus321.vsblob.vsassets.io
  • Triggering command: .dotnet/dotnet .dotnet/dotnet build test/Microsoft.TestPlatform.Filter.Source.UnitTests/Microsoft.TestPlatform.Filter.Source.UnitTests.csproj -f net9.0 --local commit.gpgsign de/node/bin/bash (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[Youssef1313]

@copilot Revert the file mode change here.

[Copilot]

Reverted in 1684c75.