Upgrade to xunit v3

[MichaelSimons]

Configuration:

• test/Directory.Build.props: Added TestRunnerName=XUnitV3, OutputType=Exe, suppressed xUnit1051
• Directory.Packages.props: Removed v2 packages, added xunit.v3.* and Verify.XunitV3

Package references updated in 8 csproj files (3 test, 3 tool, 2 utility)

Code changes across ~30 files:

• using Xunit.Abstractions; → removed/replaced with Xunit.Sdk; or Xunit.v3;
• SharedTestOutputHelper & TestOutputLogger: Added Write(), Output (new v3 ITestOutputHelper members)
• VerificationEngine.cs: Removed UseVerifyAttribute (no longer needed in v3)
• PublicAPI.Unshipped.txt: Updated type signatures
• Nullable fixes in Mock serialization code

Note: xUnit1051 (TestContext.Current.CancellationToken) is suppressed for now — 522 instances to fix incrementally.