feat: add IKeyedDataSource interface for keyed shared fixtures

[thomhurst]

Summary

• Adds IKeyedDataSource interface to TUnit.Core.Interfaces with a string Key { get; set; } property
• Sets the key inside the factory lambda in both ClassDataSources and SharedDataSources — after Activator.CreateInstance() but before IAsyncInitializer.InitializeAsync() runs
• Eliminates the need for per-key boilerplate subclasses when using SharedType.Keyed

Closes #4764

Test plan

• TUnit.Core builds with 0 errors
• TUnit.TestProject builds with 0 errors (new KeyedDataSourceTests with 4 tests)
• TUnit.Engine.Tests builds with 0 errors (new KeyedDataSourceTests engine test)
• CI: unit tests pass
• CI: engine tests pass (verifies key is set to "alpha"/"beta", key available during InitializeAsync, same key returns same instance)

🤖 Generated with Claude Code

[claude]

Code Review

I found one issue that needs attention:

Issues Found

1. Missing PublicAPI Snapshot Tests - The PR adds a new public interface IKeyedDataSource but doesn't include the required .verified.txt files from running dotnet test TUnit.PublicAPI. This is required by CLAUDE.md Rule 2.

What I Checked

✅ CLAUDE.md Compliance - Reviewed against all mandatory rules
✅ Bug Detection - Scanned for logic errors, compilation issues, and security vulnerabilities
✅ Code Quality - The implementation is clean and straightforward

Overall Assessment

The feature implementation looks solid. The IKeyedDataSource interface is well-designed with clear XML documentation, and the integration into ClassDataSources and SharedDataSources is implemented correctly. The key-setting happens at the right time (after instantiation but before InitializeAsync), and the code properly handles both generic and non-generic paths.

Once the snapshot tests are run and the .verified.txt files are committed, this will be good to merge.