Add DependencyModules.xUnit4 for xunit.v3 4.x

[ipjohnson]

This branch also carries #119, which was merged into it: ICurrentTestProvider, CurrentTest, and TestOutputLoggerProvider in DependencyModules.Testing. #119 describes that part.

What

• DependencyModules.xUnit4, a new package that compiles the DependencyModules.xUnit sources against xunit.v3 [4.0.0,5.0.0), with its own assembly name. DependencyModules.xUnit stays on [3.2.2,4.0.0).
• The code that differs between the majors is in Impl/ModuleTestCase.XunitMajor.cs: Run for each major, and CreateTest. On 4.x, CreateTest gives a row's Label and DisableParallelization to XunitTest, as [Theory] does. The discoverer needs no #if, because the same label: null call binds to the overload of each major.
• XunitMajor in Directory.Build.props selects the xunit package, the runner, and the DependencyModules project for the three xUnit test projects. scripts/test-xunit.sh 4 runs them on 4.x, in build-package and in release.
• xunit-prerelease.yaml runs them each week on the newest xunit.v3 on nuget.org, prereleases included.
• verify-packages.sh packs the new package. It now fails if a test package's framework range does not stop below the next major.
• Docs: the xUnit page gives the package for each major and a note about dotnet test on 4.x. The README, the testing overview, and CONTRIBUTING are updated.

Why

The plan in #117 was a run-time lookup for GetTestCaseDetails and a wider range. Built against 3.2.2 with that lookup, every [ModuleTest] on 4.0.1 then failed at discovery:

System.TypeLoadException: Method 'Run' in type 'DependencyModules.xUnit.Impl.ModuleTestCase' from assembly 'DependencyModules.xUnit, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation.

4.0 added ParallelMode, ExecutionScheduler, and FixtureMappingManager parameters to ISelfExecutingXunitTestCase.Run. The six-parameter XunitRunnerHelper.RunXunitTestCase that Run calls is [Obsolete(error: true)] in 4.x and throws NotSupportedException. A 3.2.2 build cannot declare the new Run, and a run-time lookup cannot supply an interface method. A Mono.Cecil pass over every xUnit reference in the 3.2.2 build found these two, the known GetTestCaseDetails overload, and one constructor that 4.x only marks obsolete.

One design keeps a single package: stop being self-executing and dispose in PostInvoke. All suites passed on both majors that way. But PostInvoke is synchronous, so it has to block on DisposeAsync. A probe service that disposes through the context it was created on hung under the Aggressive algorithm, on both majors. The awaited Run disposes the same service in 5 ms. So, as #117 says for this case, 4.x gets its own line.

Notes

• The 4.x test leg uses xunit.v3.mtp-off. The Microsoft.Testing.Platform v2 that xunit.v3 4.x brings refuses the VSTest dotnet test on the .NET 10 SDK, and xUnit ships mtp-off for that case.
• The prereleases carried each 4.0 break. The GetTestCaseDetails overload went in 4.0.0-pre.81 (2026-04-12), the old RunXunitTestCase started to throw in pre.108 (2026-05-02), and Run changed in pre.154 (2026-07-17). The weekly workflow would have failed on each.
• Hardened.Shared.Testing.xUnit 0.39 has a 4.x break of its own: its XunitCurrentTestProvider reads IXunitTestClass.Class, which 4.0 moved to ICoreTestClass. Add CurrentTest and TestOutputLoggerProvider to DependencyModules.Testing #119, now merged into this branch, gives Hardened a framework-neutral seam, so it can drop [HardenedTest].
• No CHANGELOG entry. The cut commit carries it.

Verified

• scripts/coverage.sh 85: 1069, 161, 39, and 2 tests pass on net8.0 and net10.0, at 92.3% line coverage.
• scripts/test-xunit.sh 4: 1070, 161, and 2 tests pass on both TFMs, on xunit.v3.mtp-off 4.0.1 with runner 4.0.0.
• scripts/verify-packages.sh, dotnet csharpier check ., and npm run build pass.
• The API snapshot of the 4.x build differs from the 3.x one in two lines: ICoreTestCase in the interfaces of ModuleTestCase, and the three new Run parameters.
• With Add CurrentTest and TestOutputLoggerProvider to DependencyModules.Testing #119 merged in, scripts/coverage.sh 85 passes 1081, 171, 46, and 2 tests at 92.3% line coverage, and scripts/test-xunit.sh 4 passes 1082, 171, and 2.

Closes #117.

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