Polish improvements after PR review fixes
- README.md: drop the null-forgiving operator from the usage example in favor of a nullable annotation.
- WebGpuEp.cs: throw PlatformNotSupportedException for unsupported OS or process architecture instead of silently returning placeholder strings; rewrite GetArchTag with a switch expression.
- plugin-win-webgpu-test-stage.yml: deduplicate the test csproj path via a job-level WebGpuTestProject variable; drop redundant 'clean: true' from 'checkout: self' (workspace.clean: all already covers it); tighten the .nupkg version regex to require a leading digit and add a clear error if no matching .nupkg is found.
