NuGet.Protocol nullability v2

[nkolev92]

Bug

Progress: NuGet/Home#14851

Description

Continues the NuGet.Protocol nullable annotation work. Removes #nullable disable from 8 more files and annotates public API surfaces.

Files migrated

• HttpSourceCacheContext.cs – RootTempFolder made string?
• NullSourceCacheContext.cs – fixed WithRefreshCacheTrue()/Clone() to use Instance property instead of _instance field (avoids returning null before first access)
• PackageDownloadContext.cs – DirectDownloadDirectory, ClientPolicyContext, PackageSourceMapping made nullable; directDownloadDirectory and packageSourceMappingConfiguration parameters made nullable
• RemoteSourceDependencyInfo.cs – contentUri made non-null with ArgumentNullException guard (see risk note below); fixed Equals(object) which was incorrectly casting to PackageDependencyInfo instead of RemoteSourceDependencyInfo
• SourceCacheContext.cs – Clone() now assigns _generatedTempFolder via field instead of through the GeneratedTempFolder property (which has side effects); Dispose uses captured local
• SourcePackageDependencyInfo.cs – Source, DownloadUri, PackageHash made nullable (all optional per XML docs)
• UserAgent.cs – removed #nullable disable (no annotation changes needed)
• UserAgentStringBuilder.cs – _vsInfo, _osInfo, _ciInfo fields and WithVisualStudioSKU parameter made nullable

Risk note

RemoteSourceDependencyInfo constructor now throws ArgumentNullException for contentUri. Previously null was silently accepted. The ContentUri property is typed as non-null (string!) in the public API. Callers passing null today would hit a NullReferenceException later when the value is used — the new guard makes this fail-fast instead.

Bug fix

RemoteSourceDependencyInfo.Equals(object) was casting to PackageDependencyInfo instead of RemoteSourceDependencyInfo, meaning it always returned false for valid comparisons.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

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