Let \( f(x) = \displaystyle\int_{0}^{x}\!\left(t + \sin\!\left(1 - e^{t}\right)\right)\,dt, \; x \in \mathbb{R} \). Then, \(\displaystyle \lim_{x \to 0} \frac{f(x)}{x^{3}} \) is equal to:
\( \frac{1}{6} \)
\( -\frac{1}{6} \)
\( -\frac{2}{3} \)
\( \frac{2}{3} \)