片方だけだけどこんな感じ？
テストケースをtoStrictEqualにしたら通りました

/** snake_case → camelCase オブジェクト */
export const snakeToCamelCaseObject = <T extends object>(object: T): SnakeToCamelCaseObject<T> => {
  if (isObject(object)) {
    return Object.entries(object).reduce((accumulator, [snakeKey, value]) => {
      if (isObject(value) && Array.isArray(value)) {
        accumulator[snakeToCamelCase(snakeKey)] = value.map(item => snakeToCamelCaseObject(item)) as SnakeToCamelCaseObject<T>;
        return accumulator;
      } else if (isObject(value)) {
        accumulator[snakeToCamelCase(snakeKey)] = snakeToCamelCaseObject(value);
        return accumulator;
      } else {
        accumulator[snakeToCamelCase(snakeKey)] = value;
        return accumulator;
      }
    }, {}) as SnakeToCamelCaseObject<T>;
  }
  else if (Array.isArray(object)) {
    return object.map(item => snakeToCamelCaseObject(item)) as SnakeToCamelCaseObject<T>;
  }
  return object as SnakeToCamelCaseObject<T>;
};