fix(mcts): follow tree selection through to a leaf

[liuzhengyang699]

MCTS.select stopped after choosing one child of the root. Once that child had already been expanded, later simulations expanded it again instead of selecting among its existing descendants. With one available action and three simulations, the action generator received the root once and the same first-level state twice.

Continue applying the existing UCT rule until reaching a leaf. This lets later simulations grow deeper branches and use the statistics already collected below the root, without changing the exploration formula or simulation budget.

Validation: python -m pytest tests/test_mcts_selection.py tests/test_conversation_logging_approaches.py -q — 9 passed, with no model calls. Two new regression tests fail before the fix. Ruff passed for the new test file; git diff --check passed.

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