[huxiaoyi-ovo]

Summary

The bidirectional breadth-first planner reconstructs paths from both search trees, but three cases are mishandled:

• No path: exhausted search frontiers leave both meeting nodes as None, yet reconstruction is still attempted, raising an exception instead of returning an empty path.
• Start equals goal: reconstruction indexes closed_set[-1] for the root node, raising KeyError.
• Successful path: both search halves include the meeting cell, so concatenation duplicates that waypoint.

Fix

Reconstruct each half directly using its existing parent pointers. Return ([], []) when no meeting point was found, and include the shared meeting waypoint only once. The search order, collision checks, grid representation, and public planning() signature remain unchanged.

Regression tests

A new test module covers:

1. An unreachable goal separated by an obstacle wall.
2. An adjacent goal, with each waypoint appearing exactly once.
3. Coincident start and goal, returning one waypoint.
4. A longer reachable route with contiguous steps and no repeated meeting cell.

Two files changed. No new dependencies.

Cross-platform CI validation is running on the fork.

AI disclosure: ChatGPT assisted with code analysis, the fix, and regression tests.