Fix InvalidOperationException in HttpRequestHeadersExtensions.Fill due to unsafe enumeration

[reneemundie]

This PR fixes a concurrency issue where HttpRequestHeadersExtensions.Fill() enumerates a shared IDictionary<string, string> that may be modified by another thread, resulting in:

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.

This happens during high-concurrency scenarios, like parallel calls to SaveObjectsAsync, when request headers are reused across threads.

Fix: Convert the dictionary to a list before enumeration using .ToList() to ensure thread-safe iteration.

This change does not alter behaviour but protects against intermittent runtime exceptions.

Related issue: [bug]: InvalidOperationException: Collection was modified; enumeration operation may not execute.

[github-actions]

Thanks for contributing to our API clients! Sorry to close your PR, but this repository is fully generated, you can port your changes to the API Clients Automation repository. If you need some guidance, feel free to open an issue or read our contribution guide.