linked_list.delete_ith: out-of-bounds check and tail-deletion condition are incorrect

[EvanTsai370]

In the delete_ith method of the LinkedList<T> implementation, there are two logic errors:

Bounds check allows index == length

if self.length < index {
    panic!("Index out of bounds");
}

This only panics when index > length, but permits index == length. The valid index range is 0..length, so any index >= length should panic.

Tail-deletion condition is off by one

if self.length == index {
    return self.delete_tail();
}

To delete the last element, the correct condition is index == length - 1, not index == length.