fix: AOC 2023 day 23 part 2 off by one error in DFS

[Clement-Lelievre]

hi @norvig !

I compared the results you got on the example input both with your initial attempt (154, the correct number) and with your optimized attempt (162). I also printed out the compressed graph, still on the example input, and saw it has 9 nodes. Now looking at your max_cost_graph_path function, the dfs func nested in it returns 0 if the end square has been reached else longest + 1. Here is the error and that explains why you are off by 8 (9 - 1 i.e. 9 nodes in total minus the end node). Same goes for the actual input.

Thanks for all the amazing solutions - they're always such great learning experiences! 🧩