fix: return infinity when no A* path exists

[ToB213]

Summary

Fix AStarPathPlanning.get_distance() to return float("inf") when no path exists.

Previously, an unreachable destination returned 0.0 because the distance was calculated from an empty path. This made it indistinguishable from a valid zero-distance path where the source and destination are the same.

Changes

• Return float("inf") when get_path() returns an empty list
• Preserve 0.0 for a valid path from an entity to itself

Fixes #158