fix: better max_threads calculation

[AlliBalliBaba]

I realized that when setting max_threads, the default behavior should be to minimize the amount of num_threads, in other words this configuration:

frankenphp {
    max_threads 20
    worker {
        num 5
        ...
    }
}

should start off with 5 worker-threads and 1 regular thread (not 5 worker threads and 15 regular threads that often end up being unused).

I added some tests to ensure the current num_threads logic does not change

[dunglas]

Thanks