Fix Dijkstra failure when goal is unreachable

[huxiaoyi-ovo]

Summary

• handle an exhausted open set in the Dijkstra planner
• return empty path lists instead of raising ValueError
• add a regression test with a goal separated by a solid obstacle wall

Problem

When the goal is unreachable, Dijkstra eventually exhausts open_set. The next call to min(open_set, ...) then raises ValueError: min() arg is an empty sequence.

Other grid-search implementations in this repository already guard against an empty open set. This change adds equivalent handling to Dijkstra and returns ([], []) to make the no-path result explicit.

Tests

Added test_unreachable_goal in tests/test_dijkstra.py to verify that an unreachable goal returns empty path lists without throwing an exception.