Fix build-task LibraryImport generation with extensionReceiver

[Jevan Saks (jevansaks)]

Summary

Fixes #1825.

Keep LibraryImport partial methods in a private nested partial class, outside C# 14 extension blocks. Expose ordinary forwarding methods through the configured receiver, preserving source-generated marshalling, parameter modifiers, API documentation, and non-import attributes. Normal generation without extensionReceiver is unchanged.

Reuse the existing two-project sdk-roslyn5 integration test rather than adding a new harness. Add the exact PInvoke.GetCurrentThreadId() call from the issue, plus void and pointer/friendly-overload calls. Run that same project with CsWin32RunAsBuildTask=true in both GitHub Actions and Azure Pipelines. Document build-task composition support.

Generated code: before and after

Focused excerpts for GetCurrentThreadId from the integration test's extender, with file headers, XML documentation, unrelated attributes, and other members omitted for clarity.

Before (broken)

namespace Windows.Win32
{
    public static partial class AppPInvokes
    {
        extension (global::Windows.Win32.PInvoke)
        {
            [LibraryImport("KERNEL32.dll")]
            public static partial uint GetCurrentThreadId();
        }
    }
}

The extension block is not a partial type, even though its containing class is partial. The compiler rejects the partial member (CS0751, CS8795), and the LibraryImport generator cannot provide its implementation (SYSLIB1050).

After (fixed)

namespace Windows.Win32
{
    public static partial class AppPInvokes
    {
        extension (global::Windows.Win32.PInvoke)
        {
            public static uint GetCurrentThreadId() =>
                global::Windows.Win32.AppPInvokes.AppPInvokesImports.GetCurrentThreadId();
        }

        private static partial class AppPInvokesImports
        {
            [LibraryImport("KERNEL32.dll")]
            public static partial uint GetCurrentThreadId();
        }
    }
}

The import now lives in a normal partial type, where the LibraryImport generator can implement it. The extension member has a regular forwarding body, so callers still use PInvoke.GetCurrentThreadId() without disabling source-generated marshalling.

Verification

• Reproduced before the fix: packaged the unchanged generator and built the extended integration test with build-task generation. The owner built successfully; the extender failed with SYSLIB1050, CS8795, and CS0751 for GetCurrentThreadId.
• After the fix: rebuilt and packaged the generator, then successfully built the integration test in source-generator and build-task modes and executed it on Windows.
• Also verified build-task generation with DisableRuntimeMarshalling=true, XML documentation generation, and multi-file output with a custom NativeMethods host name.
• All 22 existing ExtensionReceiverTests passed.
• Generator build and package succeeded with warnings treated as errors. Integration builds reported only the existing NU1510 package-pruning warnings.