Harden file storage internals

[dereuromark]

Summary

• fix path builder config handling for custom sanitizers, random path callbacks, and custom separators
• replace raw missing-UUID fatals with an explicit exception and document the UUID requirement
• stream storeFile() writes and add regression coverage while cleaning PHPUnit 12 test notices

Notes

• no intended BC break: this keeps the existing API and turns previous typed-property fatals into explicit runtime exceptions
• local validation passed with composer test, vendor/bin/phpstan analyze --error-format=raw, composer cs-fix, and composer cs-check