xUnit -> MSTest Migration

Migrate a .NET test project from xUnit.net (v2 or v3) to MSTest v4. The outcome is a project that:

• References MSTest v4 packages (or MSTest.Sdk 4.x) instead of xunit* / xunit.v3.*
• Has every [Fact]/[Theory] rewritten as [TestMethod] and every assertion mapped to the MSTest equivalent
• Builds cleanly with the same target framework
• Passes the same set of tests (modulo intentional changes documented below)
• Preserves the current test platform (VSTest stays on VSTest; MTP stays on MTP)

This is a cross-framework migration. Do not bundle it with a version upgrade or a platform switch in the same pass -- if both are needed, do this skill first, commit, then run migrate-mstest-v3-to-v4 (if you stopped on v3) or migrate-vstest-to-mtp.

When to Use

• The project references xunit, xunit.assert, xunit.core, xunit.extensibility.core/execution, xunit.abstractions, or any xunit.v3.* package, and you want to switch to MSTest
• You want a single .NET test framework across a solution that today mixes xUnit and MSTest

Inputs