test: migrate to xUnit v3 and run the Execution suite in CI

[StuartMeeks]

Two changes, both in the test stack. The shipped app is unaffected.

xUnit v3

The xunit package ID is the feature-frozen v2 line — 2.9.3 is its latest. v3 ships as xunit.v3, currently 4.0.0. v3 self-hosts on Microsoft Testing Platform instead of VSTest, so the whole VSTest stack comes out:

Out	In
xunit 2.9.3	xunit.v3 4.0.0
Microsoft.NET.Test.Sdk 18.9.0	— (v3 hosts itself)
xunit.runner.visualstudio 4.0.0	— (MTP replaces the adapter)
coverlet.collector 10.0.1	Microsoft.Testing.Extensions.CodeCoverage 18.10.0

Knock-on changes:

• Test projects are now executables (<OutputType>Exe</OutputType>) — a v3 test project hosts its own runner.
• A repo-root global.json opts dotnet test into the MTP runner. That runner takes the project as --project <path> rather than a bare argument, so every invocation in ci.yml, release.yml, README.md and CONTRIBUTING.md was updated.
• TRX reports come from --report-xunit-trx --report-xunit-trx-filename <name>; v3 4.0 namespaced its report switches, and --logger "trx;..." is a VSTest concept that no longer applies.

Test-code changes

The v3 analyzers (2.0.0) added two rules that TreatWarningsAsErrors turns into hard build failures. Both were fixed rather than suppressed:

• xUnit1051 (208 sites) — calls taking a CancellationToken now pass TestContext.Current.CancellationToken, so a hung await is actually interruptible by a timeout or a cancelled run. This matters most in the Execution suite, which does real process and PTY I/O. Applied mechanically via the analyzer's own code fix (dotnet format analyzers), not by hand.
• xUnit2033 (12 sites) — use the value Assert.Single returns instead of re-deriving it with [0]. Hand-applied; the fixer doesn't support fix-all. This matches the idiom already used elsewhere in these suites.

CI gap

Snipdeck.Execution.Tests was built but never run by any job — its 61 tests only ever executed on a developer machine. It now runs:

• on ubuntu, alongside Core (the job is renamed accordingly);
• on Windows too, because Execution wraps ConPTY and process launching — the ubuntu run only proves it is portable, not that it works on the platform Snipdeck ships to.

Also

Acknowledgements in README.md and the in-app Settings list swap Coverlet for Microsoft Testing Platform, matching what is actually referenced now.

Verification

• 372 tests pass on both platforms — Linux (Core 245, Execution 61, importer 66) and Windows (Core 245, Execution 61), the latter run with the exact switches CI now uses, confirming TRX artefacts are produced.
• Full solution, WinUI head included, builds clean: 0 warnings, 0 errors on a Windows 11 host.

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