Mark "sealed" and "static" where appropriate

[alxu-avpt]

This improves code readability and performance.

[jeremy-visionaid]

Thanks for the PR! I'm happy to merge it, but it raises a bigger question about why the analyzers aren't triggering for them currently. I hadn't intentionally disabled them, but they're not showing as suggestions for net10.0 for me even though that's the default. So, thanks for bringing it to my attention - It's on my mind to improve the editorconfig and StyleCop enforcement though to catch these kinds of thing.

[alxu-avpt]

Based on https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1852, I changed AnalysisLevel to latest-recommended, and set dotnet_code_quality.CA1852.ignore_internalsvisibleto = true, then the warning was triggered. Not sure if both are necessary. Thanks for quick merge :)

[jeremy-visionaid]

Yeah, I've got a local branch with AnalysisLevel set to latest-all, but CA1852 still doesn't trigger 🤔

[jeremy-visionaid]

Oh yup dotnet_code_quality.CA1852.ignore_internalsvisibleto = true does allow it to work but only at build time, not in VS Code. So, seems like some kind of upstream bug there. Thanks for the hint 👍