Enforce #nullable enable in source packages

[jjonescz]

Fixes build failures in dotnet/dotnet#8731

src\sdk\artifacts\.packages\microsoft.codeanalysis.buildclient\5.12.0-ci\contentFiles\cs\net10.0\IBuildEnvironment.cs(16,18): error CS8669: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.

And ensures they don't happen in the future.

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR adjusts Roslyn’s source-package build and packaging settings so that nullable annotations in packaged source files are always in an explicit #nullable context (per-file), preventing downstream consumers from hitting missing-nullable-context diagnostics.

Changes:

• Skip importing PackageProject.props/targets for IsSourcePackage projects under src/NuGet so those source-package projects can build under normal SDK compilation behavior.
• Add #nullable enable directives to the BuildClient source-packaged files that use nullable annotations.
• Set <Nullable>disable</Nullable> for IsSourcePackage projects to require per-file #nullable directives in the packaged sources.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/NuGet/Directory.Build.targets	Avoids importing PackageProject.targets for IsSourcePackage projects under src/NuGet.
src/NuGet/Directory.Build.props	Avoids importing PackageProject.props for IsSourcePackage projects under src/NuGet.
src/Compilers/Shared/StandardBuildEnvironment.cs	Adds #nullable enable at file scope so nullable annotations are in an explicit nullable context.
src/Compilers/Shared/IBuildEnvironment.cs	Adds #nullable enable at file scope so nullable annotations are in an explicit nullable context.
eng/targets/Imports.targets	Sets <Nullable>disable</Nullable> for IsSourcePackage projects to enforce per-file #nullable enabling.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.