Added missing methods and implemented tests

lib/data/dao/player_group_dao.dart

@@ -16,7 +16,7 @@ class PlayerGroupDao extends DatabaseAccessor<AppDatabase>
       ..where((pG) => pG.groupId.equals(groupId));
     final result = await query.get();
 
-    List<Player> groupMembers = [];
+    List<Player> groupMembers = List.empty(growable: true);
 
     for (var entry in result) {
       final player = await db.playerDao.getPlayerById(playerId: entry.playerId);
@@ -38,13 +38,38 @@ class PlayerGroupDao extends DatabaseAccessor<AppDatabase>
     return rowsAffected > 0;
   }
 
-  /// Adds a player to a group with the given [playerId] and [groupId].
-  Future<void> addPlayerToGroup({
+  /// Adds a [player] to a group with the given [groupId].
+  /// If the player is already in the group, no action is taken.
+  /// If the player does not exist in the player table, they are added.
+  /// Returns `true` if the player was added, otherwise `false`.
+  Future<bool> addPlayerToGroup({
+    required Player player,
+    required String groupId,
+  }) async {
+    if (await isPlayerInGroup(playerId: player.id, groupId: groupId)) {
+      return false;
+    }
+
+    if (await db.playerDao.playerExists(playerId: player.id) == false) {
+      db.playerDao.addPlayer(player: player);
+    }
+
+    await into(playerGroupTable).insert(
+      PlayerGroupTableCompanion.insert(playerId: player.id, groupId: groupId),
+    );
+
+    return true;
+  }
+
+  /// Checks if a player with [playerId] is in the group with [groupId].
+  /// Returns `true` if the player is in the group, otherwise `false`.
+  Future<bool> isPlayerInGroup({
     required String playerId,
     required String groupId,
   }) async {
-    await into(playerGroupTable).insert(
-      PlayerGroupTableCompanion.insert(playerId: playerId, groupId: groupId),
-    );
+    final query = select(playerGroupTable)
+      ..where((p) => p.playerId.equals(playerId) & p.groupId.equals(groupId));
+    final result = await query.getSingleOrNull();
+    return result != null;
   }
 }