4.5.12 ᄎ* Given a positive integer $n \geqslant 2$, find the largest real number $\lambda$ such that the inequality $a_{n}^{2} \geqslant \lambda\left(a_{1}+\right.$ $\left.a_{2}+\cdots+a_{n-1}\right)+2 a_{n}$. holds for any integers $a_{1}, a_{2}, \cdots, a_{n}$ satisfying $0<a_{1}<a_{2}<\cdots<a_{n}$.