assert(is_int($mixedCount) failure on admin/config. Also fix other errors

It appears that under undetermined conditions the statistics.settings value may be NULL although the Statistics module schema defines that configuration, so it should always be present and correctly typed if core Config API actually worked as expected.

Since this appears to be an exceptional condition related to a weakness in Config API when two modules by the same name coexist on a site (as is the case during the transition of Statistics from core to contrib), downgrading issue.

The pipeline for that fix surfaces many minor issues like cspell, phpcs, phpstan : fixing them all too (phpstan level 5).

Fixed, along with the various SAST warnings.

Automatically closed - issue fixed for 2 weeks with no activity.