Fix RuntimeError in bipartite-check DFS/BFS and clean up doctests (#12814)

* Fix `RuntimeError` in bipartite-check DFS/BFS and clean up doctests

* Iteration over `graph` mutated by `defaultdict` neighbours caused
  `RuntimeError: dictionary changed size during iteration`.
  – Iterate over `list(graph)` in both DFS and BFS helpers.
* Corrected `if __name__ == "__main__":` typo.
* Updated two doctests that now succeed after the fix.

All doctests now pass (`30/30`), eliminating a critical runtime failure and improving reliability of the graph algorithms.

Co-Authored-By: S. M. Mohiuddin Khan Shiam <147746955+mohiuddin-khan-shiam@users.noreply.github.com>

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

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Co-authored-by: Odio Marcelino <odiomarcelino@gmail.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>