[release/9.0] Update System.Formats.Nrbf ref sources and restore package description

[github-actions]

Backport of #108430 to release/9.0

/cc @jeffhandley

Customer Impact

• Customer reported
• Found internally

The short package description defined in the project is shown in NuGet client tooling when showing package search results. Without the description, only the package name is shown until the package is selected and the full readme can be shown. This fix restores the short package description.

This also gets the reference assembly updated to match the implementation assembly, adding the missing private struct dummy field. Without the missing field, customers would incorrectly be able to use an uninitialized struct. The reference source is now aligned with what is produced from dotnet msbuild /t:GenerateReferenceAssemblySource to improve maintainability going forward.

Regression

• Yes
• No

This is a new package in .NET 9.

Testing

A local package source was added to view the updated package in the NuGet Package Manager, seeing the package description in place. The package can be added to a project and referenced as expected.

Risk

Low. Does not affect the implementation assembly at all.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-policy-service]

Tagging subscribers to 'binaryformatter-migration': @adamsitnik, @bartonjs, @jeffhandley, @terrajobst