Fix minimumIdle option to maintain baseline idle connections

Current Behavior vs. Intended Behavior:
The minimumIdle option is intended to maintain a baseline number of connections in the pool, even when the pool isn't actively being used. For example, with

{minimumIdle: 6, connectionLimit: 10}

, the pool should maintain exactly 6 idle connections when no queries are running, allowing it to scale up to 10 connections under load.

The Problem:
The current implementation doesn't properly maintain this baseline. Instead of ensuring a minimum number of idle connections exists at all times, it only tries to maintain the minimumIdle value within the overall connectionLimit constraint during active usage.

The Goal is to fix the implementation so that minimumIdle truly maintains the specified number of idle connections in the pool at all times, regardless of current usage patterns.