Finish xunit3 update with MTP

[tebeco]

MTP = Microsoft.Testing.Platform
MTP has been created ~1+ years ago to remove legacy dependency on VsTest (especially in console and runner)

• use xunit3-mtpv2 (which xunit4 will use)
  • have to be EXE for test explicitly
  • have to tell csproj to use MTP runner
  • have to tell global.json to use mtp

[Copilot]

Pull request overview

Updates the test infrastructure to complete the xUnit v3 migration using Microsoft.Testing.Platform (MTP), including introducing a shared testing utility project and switching the test project to the MTP runner flow.

Changes:

• Added Leda.Testing project and moved/renamed shared test helpers into the Leda.Testing namespace.
• Updated Leda.Test to run via MTP (exe output + UseMicrosoftTestingPlatformRunner) and switched to xunit.v3.mtp-v2.
• Added global.json to pin the .NET SDK and opt in to the Microsoft Testing Platform runner.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Scripts/GenerateResults.cs	Uses Leda.Testing for diagnostic output generation after helper namespace/project move.
Leda.Testing/Leda.Testing.csproj	New shared testing utilities project referenced by the test project.
Leda.Testing/DiagnosticPrinter.cs	Renames namespace to Leda.Testing for reuse from tests and scripts.
Leda.Test/Leda.Test.csproj	Switches test project to MTP runner + xunit.v3.mtp-v2 and references Leda.Testing.
Leda.Test/DiagnosticTest.cs	Updates imports and theory data row construction for xUnit v3 usage.
global.json	Pins SDK and opts dotnet test into Microsoft.Testing.Platform.

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