Testing - Expand unit & integration test suite

[Codex-Crusader]

Summary

Expand the tests/ folder with more unit and integration tests focused on MCTS, model forward pass, self-play, and training pipeline edge cases.

Motivation

More tests increase confidence, make refactors safe, and help new contributors understand expected behavior.

Acceptance criteria / Definition of Done

• Add unit tests for:
  • board_to_tensor and tensor shape/value expectations.
  • MCTS selection step: given a small deterministic board, ensure selected moves are legal.
  • Legal-moves cache behavior if implemented (or ensure legal moves list matches board.legal_moves).
• Add integration tests:
  • Small self-play episode runs end-to-end with mocked model (e.g., deterministic policy) and completes without exceptions.
  • Training loop runs a single update step on synthetic data (mock or tiny buffer).
• Tests run in CI (pytest) and do not require GPU.
• Add instructions in CONTRIBUTING.md for running tests locally.

Implementation notes

• Use fixtures to create small boards and a tiny dummy model (so tests are fast).
• Mark heavier tests with pytest markers (e.g., @pytest.mark.slow) and exclude them from default CI.

Complexity

Low–Medium. Add a couple of “good first issue” tests and one integration test to start.