Fix: Ensure tempfile::TempDir lives long enough for OctopiiNode initialization

[miky-rola]

Summary

This PR addresses a bug in the client example code where the temporary directory intended for the Write-Ahead Log (WAL) was being deleted immediately upon creation due to improper scope management of the tempfile::TempDir guard

The Problem

The original code used tempfile::tempdir()?.path().to_path_buf() inline:

// Original code
let oct_cfg = OctopiiConfig {
// ...
wal_dir: tempfile::tempdir()?.path().to_path_buf(),
// ...
};

Reason for my change

In rust, the tempfile::TempDir struct manages the directory's lifecycle through RAII; When the temporary TempDir object created by tempfile::tempdir() was not bound to a variable, it was dropped immediately after the oct_cfg initialization line completed

The fix is to bind the result of tempfile::tempdir() to a local variable (temp_dir) that lives for the duration of the main function's scope

My added code

let temp_dir = tempfile::tempdir()?; // temp_dir now keeps the directory alive
let wal_dir_path = temp_dir.path().to_path_buf();

let oct_cfg = OctopiiConfig {
// ...
wal_dir: wal_dir_path,
// ...
};