test(windows): migrate xUnit v2 -> v3 across all test projects

[deblasis]

Context

Both Windows test projects currently pin xUnit v2:

• windows/Ghostty.Tests/Ghostty.Tests.csproj -> xunit 2.9.2, xunit.runner.visualstudio 2.8.2
• dist/windows/IconGen.Tests/IconGen.Tests.csproj -> xunit 2.9.2, xunit.runner.visualstudio 2.8.2 (added in # 206)

xUnit v3 is the active line. The CsWin32 review that informs the modern .NET reviewer skill explicitly notes v3 is the standard for actively maintained libraries on .NET 8+. The v2 runner model relies on shadow copy and an app-domain host that is awkward on .NET 8/9, and the v3 Exe host model sidesteps both.

Scope

Bundle both test projects into a single PR so the move is atomic and the CI / dotnet test story does not temporarily diverge between them. Touches:

• csproj package references: drop xunit + xunit.runner.visualstudio, add xunit.v3 + (if still needed) xunit.v3.runner.visualstudio
• csproj properties: OutputType=Exe on each test project (v3 host model)
• runner config: move any v2 runner settings to xunit.runner.json (e.g. \"shadowCopy\": false) if we ever need to change defaults
• source code: tighten assertion shapes that the v3 analyzers flag more aggressively than v2 (e.g. Assert.Contains(x, p) over Assert.NotEmpty(x.Where(p)), Assert.Fail(msg) over Assert.False(true, msg))

Why not now

Review on # 206 (branding + strip context menus) explicitly flagged this and deferred it. Doing the migration inside a feature PR would make that PR harder to review and would leave windows/Ghostty.Tests on v2 with a one-project inconsistency. Better as its own change.

Ordering

Land this after # 204 (AOT / trim / single-file analyzer triad) so the analyzer surface is stable before the test-host change, and any xUnit v3 analyzer warnings are attributable to the migration rather than to the trim/AOT opt-in.

Acceptance

• dotnet test windows/Ghostty.Tests/Ghostty.Tests.csproj passes under v3
• dotnet test dist/windows/IconGen.Tests/IconGen.Tests.csproj passes under v3, all existing tests still green
• No leftover xunit (v2) package references anywhere in the solution
• just test-win (if it exists) still works end to end