fix: correct ground_truth comparison in BanditEnv

[bigsawman]

Summary

• ground_truth in BanditEnv is a dict mapping button names to probabilities (e.g. {"red": 0.6, "blue": 0.3, ...}), but the final-turn winner check compares a button string directly against this dict (button == self.state.game_state['ground_truth']), which always evaluates to False.
• This means the player can never be recognized as having chosen the correct button — every game ends with an incorrect outcome and a regret-based reward.
• Fix: find the button with the highest probability via max(..., key=...), then compare the player's choice against that.

Reproduction

ground_truth = {"red": 0.6, "blue": 0.3}
button = "red"
print(button == ground_truth)  # False — string vs dict, always False

Test plan

• Verified that _regret() already correctly uses ground_truth as a dict (calls .values() and indexes by button name), confirming this is a dict, not a string.
• Confirmed the fix matches the intended semantics: reward 1.0 when the player picks the highest-probability button.