Snapshot graph nodes when checking bipartiteness

[beauagainagainagainagainagain]

Summary

• avoid runtime errors from defaultdict adjacency expansion by iterating over a snapshot of the graph's keys in both DFS and BFS checks
• remove the custom neighbor iterator helper and rely on the stored adjacency iterable directly

Testing

• pytest graphs/check_bipatrite.py

---

https://chatgpt.com/codex/tasks/task_e_68ca08d5d7a88320a6788a0fd720acce