fix(phpstan): price calculation, logging, and file handling improvements

In this commit, several enhancements are made in the ajax price calculation PHP files including
more verbose error logging and minor refactoring of the calculation itself. Instead of adding an
error message in QUI, we now add detailed error handling using QUI\System\Log.

Additionally, we have improved file handling in `ajax/output/sendMail.php` by ensuring the file is
an instance of `QUI\Projects\Media\File` or `QUI\Projects\Media\Image` before attaching it.

Lastly, we added an option for parsing in method getCustomerFiles of the ErpEntityInterface. This
commit will make the scripts more efficient and robust against errors.