Update xunit.v3 to 4.0.0 and enable full parallelization

[meziantou]

What changed

• Upgrade to xunit.v3 4.0.0 (xunit.analyzers 2.0.0 transitively) and Meziantou.NET.Sdk 1.0.143.
• Enable xunit v3 4.0.0 full test parallelization (all tests run in parallel regardless of collections).

How

xunit.v3 4.0.0 drops the VSTest bridge and defaults to Microsoft Testing Platform (MTP) v2. On the .NET 10 SDK the old VSTest path for MTP is gone, so dotnet test (used by CI) needs the MTP mode.

Meziantou.NET.Sdk.Test 1.0.143 does the heavy lifting automatically for the test project:

• Adds an implicit xunit.v3.mtp-v2 4.0.0 reference when no test framework is referenced.
• Sets OutputType=Exe (MTP test projects must be executable) and the MTP runner.
• Source-generates [assembly: Xunit.v3.Parallelization(Mode = Xunit.Sdk.ParallelMode.All)] for full parallelization.

As a result the test project needs no xunit package references, no OutputType, no UseMicrosoftTestingPlatform, and no manual parallelization attribute.

The only remaining manual bit is global.json, which opts dotnet test into MTP mode on the .NET 10 SDK:

"test": { "runner": "Microsoft.Testing.Platform" }

Notes for reviewers

• The global.json edit is normally discouraged by AGENTS.md, but the test.runner opt-in is the only supported way to run these MTP tests via dotnet test on the .NET 10 SDK (and the SDK version bump also lives in global.json).
• Local dev: under MTP, dotnet test --filter uses xunit's query filter language (/assembly/namespace/class/method), not the VSTest FullyQualifiedName~ syntax. CI runs unfiltered so it is unaffected.

Testing

• Build: clean, 0 warnings (default Roslyn + roslyn4.2).
• dotnet test full suite: 3605 passed, 0 failed, 0 skipped, with full parallelization enabled.