coturn: invalid corpus/FuzzStun.zip?

Started having this issue - FuzzStun.zip is a file in oss-fuzz corpus and not in coturn corpus

```
Step #5: Status: Downloaded newer image for gcr.io/oss-fuzz-base/base-runner:ubuntu-24-04
Step #5: gcr.io/oss-fuzz-base/base-runner:ubuntu-24-04
Step #5: [/corpus/FuzzStun.zip]
Step #5: End-of-central-directory signature not found. Either this file is not
Step #5: a zipfile, or it constitutes one disk of a multi-part archive. In the
Step #5: latter case the central directory and zipfile comment will be found on
Step #5: the last disk(s) of this archive.
Step #5: unzip: cannot find zipfile directory in one of /corpus/FuzzStun.zip or
Step #5: /corpus/FuzzStun.zip.zip, and cannot find /corpus/FuzzStun.zip.ZIP, period.
Step #5: Failed to unpack the corpus for FuzzStun. This usually means that corpus backup for a particular fuzz target does not exist. If a fuzz target was added in the last 24 hours, please wait one more day. Otherwise, something is wrong with the fuzz target or the infrastructure, and corpus pruning task does not finish successfully.
```

Start by tracing the corpus-pruning task's handling of /corpus/FuzzStun.zip and compare the oss-fuzz and coturn corpus sources. Done means the FuzzStun corpus is unpacked successfully and the pruning task completes without this error.

Written by the indexing model from the issue text.