Make Othello starting-player tests RNG-version-agnostic

[gweber]

Make Othello's starting-player-dependent tests RNG-version-agnostic

Four tests in tests/test_othello.py (test_init, test_terminated,
test_legal_action, test_observe) hard-coded the outcome of the starting-player
coin flip in Othello._init (current_player = bernoulli(key)). They picked a
specific PRNGKey — via a double jax.random.split chosen to land on
current_player == 0 — and then made absolute board/reward/observation assertions
keyed to that player.

The exact bernoulli result for a given key is not stable across jax.random
versions, so on newer JAX (e.g. 0.10.x) the flip returns 1 and all four tests
fail, even though the environment logic is unchanged and correct.

This replaces the brittle key-hunting with a small helper that searches keys for
the desired starting player:

def _init_with_current_player(player: int):
    for seed in range(1000):
        state = init(jax.random.PRNGKey(seed))
        if int(state.current_player) == player:
            return state
    raise AssertionError(...)

Every downstream assertion is unchanged and fully deterministic once the starting
player is fixed (board indices and outcomes don't depend on the RNG). No
environment/runtime code is touched — tests only.

Validation

Ran tests/test_othello.py on jax 0.4.30 and jax 0.10.1: the four tests
pass on both. (On 0.10.x a separate, unrelated test_api failure remains, caused
by an IndexError inside pgx.api_test/baseline infra — out of scope here.)