Refactor shared testing assertions into Odin.Testing core

[MarkDerman]

Summary

• Added a shared Odin.Testing library under Testing/Core and moved the common ServiceCollectionAssertions implementation there.
• Introduced IAssertionAdaptor plus framework-specific adapters for NUnit, xUnit v3, and xUnit v2.
• Kept each utility project as a thin facade so existing services.AssertServiceRegistration(...) call sites continue to work.
• Added the shared testing project to Odin.sln and the publish workflow so it is built and packed with the other Testing packages.

Testing

• Built Testing/Core, Testing/NUnitUtility, Testing/XUnitUtility, and Testing/XUnitV2Utility successfully.
• Ran Logging/Tests and CQRS/Tests against net10.0; both passed.