Maintain dependencies, MSTest, and package version integrity

[RejectKid]

What changed

• migrate all 307 tests from xUnit to Microsoft's MSTest 4.3.2 framework and adapter
• update Microsoft.NET.Test.Sdk to 18.8.1 and remove every xUnit package, analyzer, attribute, and assertion
• remove the unused Coverlet collector
• update actions/checkout to v7 and actions/setup-dotnet to v6
• derive CI prerelease versions from the project version
• build release and CI assemblies with the exact NuGet version and disable incremental compilation for versioned builds
• validate the package ID/version and the assembly, file, and product versions of both shipped DLLs before artifacts can publish

Why

The release workflow previously supplied PackageVersion only during packing. A future release tag could therefore produce a correctly named NuGet package containing DLLs stamped with the older project version. Incremental compilation could also reuse DLLs built with a previous version. The new package gate examines the actual DLLs inside the .nupkg and rejects either mismatch.

The test suite now uses the Microsoft test stack consistently, eliminating xUnit and its analyzer/runner dependencies.

Impact

There are no runtime API or wire-format changes. Published OpenDisNet remains dependency-free. The changes affect test tooling and release/CI integrity only.

Validation

• confirmed no xUnit references remain anywhere in source or lock files
• generator freshness verification passed
• formatting verification passed
• Release build passed with zero warnings, including MSTest analyzers
• all 307 migrated MSTest cases passed
• stable package 1.0.2 validated with matching DLL assembly/file/product versions
• prerelease package 1.0.2-ci.999 validated with matching DLL metadata
• packed-package consumers passed on .NET 9 and .NET 10
• all direct NuGet dependencies are current and the vulnerability audit is clean