migrate-xunit-to-xunit-v3

# xunit.v3 Migration Migrate .NET test projects from xUnit.net v2 to xUnit.net v3. The outcome is a solution where all test projects reference `xunit.v3.*` packages, compiles cleanly, and all tests pass with the same results as before migration. ## When to Use - Upgrading test projects from `xunit` (v2) packages to `xunit.v3` - Resolving compilation errors after updating xunit package references to v3 ## When Not to Use - Migrating between test frameworks (e.g., MSTest or NUnit to xUnit.net) — different effort entirely - Migrating from VSTest to Microsoft.Testing.Platform — use `migrate-vstest-to-mtp` - The projects already reference `xunit.v3` — migration is done ## Inputs | Input | Required | Description | |-------|----------|-------------| | Test project or solution | Yes | The .NET project or solution containing xUnit.net v2 test projects | ## Workflow > **Commit strategy:** Commit after each major step so the migration is reviewable and bisectable. Separate project file changes from code changes. ### Step 1: Identify xUnit.net projects Search for test projects referencing xUnit.net v2 packages: - `xunit` - `xunit.abstractions` - `xunit.assert` - `xunit.core` - `xunit.extensibility.core` - `xunit.extensibility.execution` - `xunit.runner.visualstudio` Make sure to check the package references in project files, MSBuild props and targets files, like `Directory.Build.props`, `Directory.Build.targets`, and `Directory.Packages.props`. ### Step 2: Verify compatibil...