Fix Mahjong hu check for exposed sets without pair

[Jiang020609]

This fixes a Mahjong hu judgment edge case.

Previously, MahjongJudger.judge_hu returned True whenever len(player.pile) >= 4.
That can mark a hand as winning even when the concealed remaining tiles do not
contain a pair.

Example: after four exposed melds, a remaining hand like [dots-6, bamboo-4]
was judged as hu.

This patch removes the early win return and requires the existing pair-check
path to validate the remaining concealed hand.

Tests added:

• four exposed sets without a pair should not be hu
• four exposed sets with a pair should be hu

Local test:
python -m pytest tests/games/test_mahjong_game.py tests/envs/test_mahjong.py

Result:
17 passed