Remove NuGet.Core package dependency

[brettfo]

This PR will allow dependabot to update dependabot again :)

Replace all usages of the NuGet.Core (v2) package with local implementations. That package is a transitive dependency of the NuGet.Client submodule and the assembly targets net40-Client which is technically incompatible with the target framework used by dependabot. Because of that, <NoWarn>NU1701</NoWarn> was present telling NuGet to ignore target framework compatibility checks. This was a very specific case where the few types necessary from that assembly were compatible with .NET Core, but by having that suppression, it made it so that dependabot couldn't update dependabot because when determining package compatibility, we do not take into consideration NU1701 because we have no way of knowing what packages that was for or in what circumstances it would be considered OK so we have to consider any target framework incompatibility as fatal.

Various documents were updated to help future maintainers and bots understand the limitations.

[Copilot]

Pull request overview

This PR removes the legacy NuGet.Core (v2) package dependency from the NuGet helper projects by replacing the few required types/usages with local implementations and by vendoring/stubbing upstream NuGet.CommandLine sources that previously depended on extern alias CoreV2. It also documents the NU1701 suppression limitation to clarify why Dependabot treats target framework incompatibilities as fatal.

Changes:

• Removed NuGet.Core package references and the NU1701 suppression from NuGetUpdater build configuration.
• Introduced local equivalents for NuGet runtime binding-redirect types (IAssembly, AssemblyBinding) and updated redirect logic to use them.
• Added/updated documentation and maintainer guidance for vendored NuGetProjects/ sources and NU1701 behavior.

Show a summary per file

File	Description
nuget/README.md	Documents NU1701 suppression as a known limitation for Dependabot NuGet updates.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Updater/IAssembly.cs	Adds local IAssembly abstraction to replace NuGet.Core runtime interface usage.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Updater/AssemblyBinding.cs	Adds local AssemblyBinding implementation for binding redirect parsing/serialization.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Updater/BindingRedirectResolver.cs	Removes NuGet.Core alias usage and returns local AssemblyBinding instances.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Updater/BindingRedirectManager.cs	Switches redirect management from NuGet.Core runtime types to local AssemblyBinding.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/NuGetUpdater.Core.csproj	Drops the NuGet.Core package dependency.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core.Test/StreamExtensions.cs	Adds a small test helper for reading streams to byte arrays.
nuget/helpers/lib/NuGetUpdater/NuGetProjects/README.md	Adds maintenance rules for the vendored NuGetProjects code (incl. removing NuGet.Core).
nuget/helpers/lib/NuGetUpdater/NuGetProjects/NuGet.CommandLine/NuGet.CommandLine.csproj	Removes NuGet.Core dependency and excludes upstream files that relied on CoreV2.
nuget/helpers/lib/NuGetUpdater/NuGetProjects/NuGet.CommandLine/Program.cs	Adds a local replacement for upstream NuGet.CommandLine program entrypoint without CoreV2.
nuget/helpers/lib/NuGetUpdater/NuGetProjects/NuGet.CommandLine/SettingsCredentialProvider.cs	Adds a local replacement provider (removing legacy dependencies).
nuget/helpers/lib/NuGetUpdater/NuGetProjects/NuGet.CommandLine/Commands/Command.cs	Adds a local replacement for upstream command base without CoreV2 usage.
nuget/helpers/lib/NuGetUpdater/NuGetProjects/NuGet.CommandLine/Commands/ProjectFactory.cs	Adds a local replacement for upstream project factory without CoreV2 usage.
nuget/helpers/lib/NuGetUpdater/NuGetProjects/NuGet.CommandLine/Commands/UpdateCommand.cs	Adds a local replacement for upstream update command without CoreV2 usage.
nuget/helpers/lib/NuGetUpdater/NuGetProjects/Directory.Build.props	Removes NU1701 suppression in the vendored projects build.
nuget/helpers/lib/NuGetUpdater/Directory.Packages.props	Removes centralized NuGet.Core package version entry.
nuget/helpers/lib/NuGetUpdater/Directory.Build.props	Removes global NU1701 suppression for NuGetUpdater projects.
.github/instructions/nuget-projects.instructions.md	Adds GitHub instructions/checklist for maintaining NuGetProjects/ after submodule updates.

Copilot's findings

• Files reviewed: 18/18 changed files
• Comments generated: 2

[sebasgomez238]

A couple of the copilot suggestions seem reasonable, feel free to ping for approval once addressed or ignored

[albertoblue87-netizen]

Done