feat: add hand detail API and enrich hand summary fields

- HandSummary: add hole_cards, starting_stacks, ending_stacks, pot_contributions
- Engine: capture all players' hole cards (not just showdown), pre/post hand stacks, per-level pot contributions
- Server: new GET /game/<game_id>/hands/<hand_number> route
- Service: add get_hand_state() method
- Tests: add ServerTests for new endpoint, update existing tests
- Existing GET /game/<game_id> auto-inherits new fields via shared to_dict()

texas_holdem/service.py

@@ -78,6 +78,13 @@ class GameManager:
     def get_game_state(self, game_id: str) -> dict[str, object]:
         return self.get_game(game_id).snapshot_completed()
 
+    def get_hand_state(self, game_id: str, hand_number: int) -> dict[str, object]:
+        state = self.get_game_state(game_id)
+        for hand in state.get("hands", []):
+            if hand.get("hand_number") == hand_number:
+                return hand
+        raise KeyError(f"hand not found: {game_id} #{hand_number}")
+
     def list_games(self) -> list[dict[str, object]]:
         with self._lock:
             games = list(self._games.values())