Add end-to-end smoke tests for the CsWinRT NuGet package

[Sergio0694]

Summary

Adds minimal, isolated end-to-end smoke tests that consume the real Microsoft.Windows.CsWinRT NuGet package — a consumption app and an authoring component — to verify the package works correctly in a real production environment, independent of the repository's own build infrastructure.

Motivation

The CsWinRT 3.0 build pipeline is complex (reference projections, forwarder assemblies, several post-build generators, and the dual reference/implementation WinRT.Runtime assembly added in this branch). The existing unit and functional tests all build against the local build infrastructure (SimulateCsWinRTNugetReference, project references to the in-repo projections, etc.), so they never exercise the actual packaged ref/lib assemblies, the packaged generators, or the NuGet build targets the way a real customer consumes them. These smoke tests close that gap: they reference the packed NuGet package exactly like an external app or component author would, validating that the correct assemblies are referenced and that all generators run correctly end-to-end.

Changes

The smoke tests live under src/Tests/SmokeTests/ and are intentionally kept out of cswinrt.slnx (the package they consume only exists after the build packs it).

• src/Tests/SmokeTests/Directory.Build.props / Directory.Build.targets: blank files that isolate the smoke tests from the repository build infrastructure, so they inherit nothing from src/Directory.Build.*.
• src/Tests/SmokeTests/Directory.Packages.props: disables central package management for the smoke tests (and shadows the repository Directory.Packages.props), so each project pins the CsWinRT package version directly.
• src/Tests/SmokeTests/.gitignore: ignores the default bin/obj output (these projects use the default output folders rather than the repo _build redirect).
• src/Tests/SmokeTests/Consumption/: an Exe that calls JsonObject.Parse(...) then Stringify() from Windows.Data.Json, exercising the Windows SDK projection, the interop generator, and the WinRT.Runtime ref/impl assemblies.
• src/Tests/SmokeTests/Authoring/: a CsWinRTComponent library exposing a minimal Greeter class, exercising WinMD generation, the reference projection, and the forwarder assembly.
• src/Tests/SmokeTests/run-smoke-tests.ps1: builds and runs the consumption test (asserting a clean exit code), then builds the authoring test and verifies that the generated Authoring.winmd is a valid Windows Runtime metadata file defining Authoring.Greeter.
• src/build.cmd: runs the smoke tests after the local nuget pack step (x64 only; skippable via cswinrt_run_smoke_tests=false).
• build/AzurePipelineTemplates/CsWinRT-PublishToNuGet-Steps.yml: runs the smoke tests after the CI NuGet pack step, against the freshly packed package.

Both projects override RestoreSources to restore the package being tested from the local build output (or the CI pack output) and everything else — notably the preview Windows SDK ref pack — from public NuGet. They use the real .NET SDK targeting pack via a pinned WindowsSdkPackageVersion and the .1 Windows TFM revision, mirroring src/WinRT.Internal. The package version and source default to the local build.cmd x64 Release output and are overridden by the build or CI that produced the package.

Validation

Verified end-to-end against a real CsWinRT 3.0 package using the committed configuration: the consumption test builds and runs (round-tripping the JSON to {"a":42} with a clean exit), and the authoring component's generated Authoring.winmd is confirmed to define Authoring.Greeter. In CI the same runner executes against the freshly packed package.