Optimize 2019 Day 18 with A* and fix grid stride bug

[google-labs-jules]

Optimized the solution for Advent of Code 2019 Day 18.
Key changes:

1. A Algorithm*: Replaced the Dijkstra search with A*. Implemented a heuristic that estimates the remaining cost by summing the maximum distance from each robot to its furthest uncollected assigned key. This significantly prunes the search space.
2. Bug Fix: Corrected the stride used for the visited array in KeyGraph::new BFS (was using height, now using width).
3. Cleanup: Removed the unused priority_queue import.
4. Verification: Added a unit test with a standard example to ensure correctness.

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PR created automatically by Jules for task 14614951169114204466 started by @viandoxdev

[google-labs-jules]

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[viandoxdev]

Whatever going to test locally anyways (and cleanup)