feat: Exclude generated attributes from code coverage

[skarllot]

What’s changed

Two internal attribute types in RegisterAttributesExtensions.cs are now marked as generated code and excluded from code-coverage analysis.

• GeneratedCode attribute signals to tools that these types are auto-generated.
• ExcludeFromCodeCoverage ensures coverage reports (e.g. Coverlet, Visual Studio) skip these classes.

Why

Coverage tools can skew metrics by including boilerplate attribute definitions that aren’t hand-written or relevant to business logic. By marking them as generated and excluding them, we:

• Keep overall coverage percentages meaningful
• Avoid noise when tracking real coverage gaps
• Follow best practices for generated code hygiene

How to verify

1. Run your coverage suite (e.g. dotnet test /p:CollectCoverage=true).
2. Observe that neither of the two attribute classes appears in the coverage report.
3. Confirm no other tests or behavior have regressed.