Move the whole repo to xunit.v3 4.x on Microsoft.Testing.Platform 2 (root global.json test runner) #4979

Decided 2026-10-12. To be done today, as its own PR after the Bobcat 0.60.0 batch PR merges (one Wolverine PR at a time).

The batch moves only the four Bobcat.Xunit projects to xunit.v3 4.0.2 (central VersionOverride + scoped global.json). xUnit 4.0 drops official support for platform v1, so the rest of the repo should follow:

• xunit.v3 3.2.2 → 4.x everywhere; decide what to do about the xUnit v2 (xunit 2.9.3) projects: move them or leave them on VSTest;
• a root global.json "test": {"runner": "Microsoft.Testing.Platform"}; then remove the scoped global.json files and XunitV3ForBobcatVersion;
• every dotnet test call in build/Build.cs, the GitHub workflows and docs becomes the --project/--solution form; check filters (--filter → MTP filters);
• code migrations: CollectionBehavior(DisableTestParallelization) → Parallelization(Mode = None), TestCaseOrderer → TestMethodOrderer where method order matters, IsTestingPlatformApplication=false on helper projects that reference the platform or xunit core;
• NoParallelization.cs / CollectionPerAssembly conventions checked under xUnit 4's parallel modes;
• CI-impact review: every CI job's test step.

Reference: Bobcat's docs/xunit.md, "Moving to xUnit v3 4.x", and Stoat's move (stoat a367cc6).

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