Fetch MR diff via the API to stop session auth issues to block diff retrieval.

What does this merge request do and why?

1. When running the vulnerability resolution using a remote GDK setup, we hit an auth issue as it's trying to navigate through the web UI but getting blocked by the sign in page. See an example of this here.
2. Retry when receiving an empty code patch, as we are encountering samples where the diff is not quite ready at the time of retrieval.
3. This MR changes the retrieval of the MR diff to go via the API instead as that seems more reliable plus we already have a token handy. Working example of a run using this branch is here.
4. When rebasing the latest main I could not execute anything as I consistently hit issues with the way we passed executor params, so this is reworked as well.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.