This is super insightful, and I think there is at least part of what you are thinking of: an abstract syntax tree! Or at the very least one could include metadata about the token under scrutiny (similar to how most editors can show you git blame / number of references / number of tests passing in the current code you are looking at...)

It makes me think about things like... "what if we also provided not just the source code, but the abstract syntax tree or dependency graph", or at least the related nodes relevant to what code the LLM wants to change. In this way, you potentially have the true "full" context of the code, across all files / packages / whatever.