Return empty path when road-map Dijkstra cannot reach goal

[huxiaoyi-ovo]

Summary

• return ([], []) when the shared road-map Dijkstra search exhausts its frontier
• add a deterministic regression test for an unreachable goal

Problem

PathPlanning/VoronoiRoadMap/dijkstra_search.py is shared by the Voronoi and visibility road-map planners. When no path exists, the search currently breaks out of the loop and then calls generate_final_path() with the untouched goal node. That produces a singleton [goal] path, which can be mistaken for a successful plan.

This change makes the no-path result explicit and consistent with other planners in the repository by returning empty coordinate lists as soon as the open set is exhausted.

Tests

Added tests/test_voronoi_dijkstra_search.py with a disconnected three-node graph and asserts that an unreachable goal returns empty path lists.