test(algorithms): add path ranking unit tests

Test coverage for information-theoretic path ranking:
- Basic path finding and ranking
- Geometric mean score computation
- Length penalty parameter (lambda)
- getBestPath() convenience function
- createPathRanker() factory with cached MI
- Configuration options (maxPaths, pre-computed cache)
- Error handling for invalid inputs
- Directed vs undirected graph behaviour
- Type preservation in ranked paths

Author: Mearman