GitHub Copilot Edit can do the second version of this. It is pretty good at it too. It sometimes gets things wrong but for your average code (and candidates typing in "C# read JSON from file" are way below average unless they never written in C#), if you give all the files for a specific self-contained part of the program, it can extend/modify/test/etc. it impressively well for an LLM.

The difference compared to where we were just 1-2 years ago is staggering.

Edit: the above is with Claude-3.5-Sonnet