Replace Dijkstra with BFS for unit-weight shortest_paths

[hongjun7]

Issue #, if available:

N/A

Description of changes:

• Replaced Dijkstra-based shortest path search O(E log V) with Breadth-First Search O(V + E) for unit-weight edges.
• Switched from a priority queue to collections.deque and simplified traversal accordingly.
• Preserved existing behavior: supports multiple sources/targets, optional max_distance, and returns paths as lists of (src, rel, dst) triplets.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[cmavro]

Thanks @hongjun7 for the PR!

I will run some qualitative tests on my side, good to see the latency improvement!