feat: Add unit tests in Core to increase coverage

[Kryptos-FR]

PR Details

Improve the code coverage of Stride.Core.CompilerServices, Stride.Core.Design and Stride.Core.Mathematics.

Details

Tests were written with the help of GitHub Copilot. It also helped detect some issue in Core.Mathematics.

One is already tracked #2672. Fixing it would potentially be a breaking change, so it wasn't done in this PR.

Another was that Plane.Negate(Plane) wasn't negating the plane, but instead normalizing it, likely because of an unfortunate copy/paste. I decided to fix it since it was never correct before.

HybridDictionary also has a few issues, like throwing when initializing with a zero capacity (which should be allowed).

Overall, we now have a very good coverage for Mathematics and CompilerServices, and a passable coverage for Design:

Assembly	Line coverage	Branch coverage
Stride.Core.CompilerServices	85.6%	74.1%
Stride.Core.Design	55.8%	56.4%
Stride.Core.Mathematics	81.3%	61.5%

Before that PR, the coverage was:

Assembly	Line coverage	Branch coverage
Stride.Core.CompilerServices	84.7%	73.2%
Stride.Core.Design	36.8%	33.5%
Stride.Core.Mathematics	16.8%	7.9%

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.