fix(mahjong): allow chow without pong/gong and relax hu pair

[Shuo-O]

Summary

• Enable chow availability when no pong/gong is triggered after a discard.
• Allow HU to use a pair taken from a triplet (count >= 2), which matches standard pair selection.

Issues fixed

1. Chow was skipped unless a pong/gong happened first.
2. HU was rejected when the only pair could be taken from a 3/4-of-a-kind.

Changes

• rlcard/games/mahjong/round.py
  • After a discard with no pong/gong, check judge_chow before dealing to the next player.
• rlcard/games/mahjong/judger.py
  • Allow pair candidates with count >= 2 and remove only two tiles as the pair.

Tests

• tests/games/test_mahjong_regressions.py
  • test_chow_available_without_pong_gong
  • test_hu_allows_pair_from_triplet

Notes

• Rules remain simplified (no yaku/score handling); this only fixes legality detection.