Fix CodeBehind NuGet config placement

[jonathanpeppers]

---

CodeBehindTests passed a file path to CopyNuGetConfig, which expects a directory and appends NuGet.config. This placed the repository configuration in a nested path, so generated projects could miss it and contact external package feeds. Pass the temporary project root instead.

• Useful description of why the change is necessary.
• Links to issues fixed: N/A
• Unit tests: Focused CodeBehind tests require an in-tree local SDK, which was unavailable. git diff --check passed.

[Copilot]

Pull request overview

Fixes incorrect NuGet.config placement in CodeBehindTests by passing a directory (project temp root) to CopyNuGetConfig, which appends NuGet.config internally. This ensures generated test projects consistently pick up the repo NuGet configuration and avoid accidentally restoring from external feeds.

Changes:

• Update CodeBehindTests to pass tempRoot (directory) instead of Path.Combine (tempRoot, "NuGet.config") (file path) to CopyNuGetConfig.

[jonathanpeppers]

@dalexsoto review

[dalexsoto]

The NuGet configuration is now copied to the correct test root, restoring CI feed discovery without introducing a product or test-path regression.