Fix nan_random_v0 ignoring observation action masks

[Doribelove]

nan_random_v0 checks for "action mask" in the observation but then reads obs["action_mask"]. Observations using the standard underscore key therefore fall through to unmasked sampling. A NaN replacement can be illegal even when the observation identifies legal actions.

Change the guard to "action_mask", restoring the existing masked-sampling branch described in the wrapper documentation. Add two deterministic AEC regression cases with a single legal action; the seeded unmasked action space selects an illegal action in both cases. This addresses the NaN replacement path, which the existing normal-action API tests do not exercise.

For an end-to-end reproduction with PettingZoo's Connect Four:

import numpy as np
from pettingzoo.classic import connect_four_v3
from supersuit import nan_random_v0

env = nan_random_v0(connect_four_v3.env())
env.reset(seed=0)
for _ in range(6):
    env.step(5)  # Fill column 5 without ending the game.
env.action_space(env.agent_selection).seed(0)
env.step(np.nan)
print(env.terminations)
env.close()

Before the fix, unmasked sampling selects the full column and terminates the game with an illegal-move penalty. After the fix, a legal column receives a piece and both termination flags remain false.

Validation on base adf4114d0b437ba4607bd329ad32e34667c0a10b, using Python 3.12.14, NumPy 2.5.3, Gymnasium 1.3.0, and PettingZoo 1.27.0:

• New regressions against the original implementation: 2 failed; with the fix: 2 passed.
• AEC/Gym mock and unwrapped tests, parallel wrapper tests, generated-agent tests, and the new regressions: 77 passed, 39 skipped.
• Existing non-Atari NaN-wrapper API tests: 120 passed.
• Pinned pre-commit hooks on both changed files, git diff --check, and wheel/sdist builds passed.

The two new regressions are included in the 77-test result. The full suite, Atari ROM tests, and the complete CI platform/Python matrix were not run locally.

AI assistance: Codex prepared the implementation, tests, and this description, and executed the reported local validation.