Generated paths to cache entries can become too long

[rogerdahl]

DeX builds paths such as /home/pasta/dex-cache/<safe name of csv>/eml.etree and if the csv name is too long, the filename becomes too long, and the operation fails. This is mostly an issue with requests from ezEML, because the submitted CSV filename is already expanded with additional information to make it unique. E.g.,

https-ezeml.edirepository.org-user-data-H-C3-A9ctor_J._Esparra-Escalera-39e8d3dc33d4d5a9f5509b63cfcf6a27-uploads-Long-Term-20Nitrogen-20and-20Phosphorus-20Dynamics-20and-20Macroinvertebrate-20Assemblages-20in-20Temperate-20Streams-nutrientsMI_rawdata_27DEC2025.csv

To fix this, we will check the length of generated filenames, and if they're longer than 256 bytes, we will truncate them and replace the truncated section with a hash.