[christopherread]

Fixes #75. Merge after #80 and #81: this branch includes their commits (cherry-picked, so history stays linear), because the xUnit v3 fixes touch the same test lines. The diff includes their changes until they merge.

Replaces #100. That branch merged #80 and #81 in, and GitVersion 6's Mainline strategy crashes on those merge commits (InvalidOperationException in MainlineVersionStrategy). The files here are identical.

Changes

• Packages: xunit 2.9.3 (deprecated) → xunit.v3 4.0.2 in Directory.Packages.props and all 9 test projects. xunit.runner.visualstudio 4.0.1 and Microsoft.NET.Test.Sdk stay, so IDE test explorers keep working.
• Projects: test projects are now <OutputType>Exe</OutputType>, as xUnit v3 requires.
• Code: using Xunit.Abstractions; is removed; ITestOutputHelper is now in the Xunit namespace.
• xUnit1051: v3's analyzer flagged 11 Task.Delay and CloseAsync calls in the broker tests that should pass TestContext.Current.CancellationToken. Fixed.
• Test runner mode: xunit.v3 runs on Microsoft.Testing.Platform (MTP), and the .NET 10 SDK refuses to run MTP projects through the old VSTest-mode dotnet test. Microsoft documents the old route as legacy. So a new global.json opts into MTP mode with "test": { "runner": "Microsoft.Testing.Platform" }. It has no sdk section, so it doesn't pin an SDK version.
• CI unit-test step:
  • The filter becomes dotnet test --solution Obvs.sln … -- --filter-not-trait "Category=Explicit" --ignore-exit-code 8.
  • MTP reports exit code 8 when a project runs zero tests, which is the case for the transport test projects that contain only Explicit tests.

For developers

dotnet test syntax changes in MTP mode:

• --filter Category=Explicit → -- --filter-trait "Category=Explicit"
• dotnet test X.csproj → dotnet test --project X.csproj
• dotnet test X.sln → dotnet test --solution X.sln

⚠️ Interaction with #82

#82 adds a CI step that runs the broker tests with the old syntax (dotnet test Obvs.RabbitMQ.Tests --filter Category=Explicit). Whichever of #82 and this PR merges second needs that step converted to:
dotnet test --project Obvs.RabbitMQ.Tests --no-build --configuration Release -- --filter-trait "Category=Explicit" (and the same for NATS and Kafka). I'll update whichever branch is left.

Testing

• Build: a full rebuild gives 0 warnings, 0 errors.
• Unit tests: all 124 pass in MTP mode with the CI command. That's the same count as before, so the trait filter excludes exactly the Explicit tests.
• NetMQ socket tests (--filter-trait "Category=Explicit"): passed in 10 of 11 runs. One run had a single failure that didn't recur in the next 8 runs. These tests rely on fixed Task.Delay timings.

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