Flag badly encoded documents

[bernhardreiter]

Do we detect badly encoded documents and treat them accordingly?

ISDuBA/ISDuBA#1470 has the example document opensuse-su-2025_14737-1.json which contains an invalid characters, so it is not UTF-8 encoded.

Go' json parser changes those characters within strings to the unicode replacement character, but does not raise an error.

• check if our downloader, checker and validator will accept those documents without warning or error.
• decide what would be a good behaviour? E.g. download acceptance by default, checker reject by default?
• estimate and then potentially implement an improvement.