Fix generator crash composing plugin under long paths with scoped package names

[Copilot]

The C# generator crashed when composing a plugin whose node_modules package directory resolved to a long path. Scoped package names like @azure-typespec/http-client-csharp-mgmt retained their forward slash on Windows, producing a mixed-separator path (...\node_modules\@azure-typespec/http-client-csharp-mgmt). Once long enough to trigger the runtime's \\?\ extended-length prefix — which requires canonical backslash separators — Directory.EnumerateFiles failed with "The filename, directory name, or volume label syntax is incorrect."

Changes

• GeneratorHandler.AddPluginDlls: replaced Path.Combine(rootDirectory, NodeModulesDir, package) with a new GetPackageDirectory helper that splits scoped package names on / into separate path segments, normalizing separators to the platform separator.
• Tests: added coverage for scoped and unscoped package directory resolution.

internal static string GetPackageDirectory(string rootDirectory, string package)
{
    var segments = new List<string> { rootDirectory, NodeModulesDir };
    segments.AddRange(package.Split('/'));
    return Path.Combine(segments.ToArray());
}

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-csharp@11218

commit: 1afd3f2