Add unit tests for BFS construction module

[Pratz1337]

The _src/se/construction/bfs.py module is critical for game tree generation but currently lacks unit test coverage. This makes refactoring risky and debugging difficult.

I propose adding a comprehensive test suite to verify get_child, add_terminal_node, and the main bfs execution loop.

[shreed27]

Thanks for adding tests for this — happy to help review or add additional edge-case coverage if needed.