Code clean up and improvements

[marcschier]

Proposed changes

PR Classification

Code cleanup and modernization to align with latest C# and .NET best practices, and to remove legacy .NET Standard 2.0 support.

PR Summary

This pull request modernizes the OPC UA .NET codebase and tests, removes .NET Standard 2.0 support from certificate/security code, and updates test code to use C# 12 and .NET 8+ features for clarity and maintainability.

• Certificate and BouncyCastle interop files (e.g., CertificateBuilder.cs, X509Extensions.cs, X509SignatureFactory.cs) are refactored to drop NETSTANDARD2_0 support and use modern .NET APIs and conditional compilation.
• Test code across the codebase is updated to use default, strongly-typed Parse methods, collection expressions, and improved async/await patterns.
• Exception handling and assertions in tests are modernized, and test method naming is made consistent with Async suffixes.
• The project and build files are updated to clarify Roslynator analyzer usage and remove obsolete netstandard tags.
• General code and test cleanup includes improved null safety, type safety, and removal of deprecated or unused code.

Types of changes

What types of changes does your code introduce?
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• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[romanett]

not familiar with this area, but why was this removed? not needed any more?

[marcschier]

It was not used, you cannot fail for missing description, because it was set to null in the configuration.

[romanett]

Is this really an OPC UA semantic?
Not shure why it is added here

[marcschier]

I needed to pull this into the interface so it can be pulled into condition state and audit event state from system context (without these areas to understand "user tokens").

[romanett]

Added in this PR for a reason?

[marcschier]

No reason - in fact we can likely remove this although, let me do that in the next PR if there are no glaring other issues.

[romanett]

should this be changed in ModelCompilerRepo as well or is this only present in UANetstandard?

[marcschier]

It is already changed there, this is the copy