Issue accessing `libwebp` corpus, possibly due to `@` in fuzz target name

I'm having some trouble accessing the `libwebp` public corpus.

Running:
```
$ python3 infra/helper.py download_corpora --public libwebp
```

Gives:
```
% python3 infra/helper.py download_corpora --public libwebp
INFO:__main__:Downloading public corpus
INFO:__main__:Downloading corpora for libwebp project to /Users/harrison/cmu/oss-fuzz/build/corpus/libwebp.
--2026-02-26 13:39:22-- https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_simple_api/public.zip
--2026-02-26 13:39:22-- https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_advanced_api/public.zip
--2026-02-26 13:39:22-- https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_demux_api/public.zip
--2026-02-26 13:39:22-- https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_webp_animencoder/public.zip
--2026-02-26 13:39:22-- https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_animation_api/public.zip
--2026-02-26 13:39:22-- https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_webp_enc_dec/public.zip
Resolving storage.googleapis.com (storage.googleapis.com)... Resolving storage.googleapis.com (storage.googleapis.com)... Resolving storage.googleapis.com (storage.googleapis.com)... Resolving storage.googleapis.com (storage.googleapis.com)... Resolving storage.googleapis.com (storage.googleapis.com)... Resolving storage.googleapis.com (storage.googleapis.com)... 2607:f8b0:4004:c21::cf2607:f8b0:4004:c21::cf2607:f8b0:4004:c21::cf, , 2607:f8b0:4004:c21::cf, 2607:f8b0:4004:c23::cf2607:f8b0:4004:c23::cf, 2607:f8b0:4004:c23::cf, , 2607:f8b0:4004:c23::cf, 2607:f8b0:4004:c07::cf2607:f8b0:4004:c21::cf, , ...2607:f8b0:4004:c21::cf, 2607:f8b0:4004:c07::cf
, 2607:f8b0:4004:c23::cf, ...2607:f8b0:4004:c23::cfConnecting to storage.googleapis.com (storage.googleapis.com)|2607:f8b0:4004:c21::cf|:443... 2607:f8b0:4004:c07::cf, 2607:f8b0:4004:c07::cf
, , ...2607:f8b0:4004:c07::cf, ...
2607:f8b0:4004:c07::cf
, ...Connecting to storage.googleapis.com (storage.googleapis.com)|2607:f8b0:4004:c21::cf|:443... , ...Connecting to storage.googleapis.com (storage.googleapis.com)|2607:f8b0:4004:c21::cf|:443...

Connecting to storage.googleapis.com (storage.googleapis.com)|2607:f8b0:4004:c21::cf|:443... Connecting to storage.googleapis.com (storage.googleapis.com)|2607:f8b0:4004:c21::cf|:443... Connecting to storage.googleapis.com (storage.googleapis.com)|2607:f8b0:4004:c21::cf|:443... connected.
connected.
connected.
connected.
connected.
connected.
HTTP request sent, awaiting response... HTTP request sent, awaiting response... HTTP request sent, awaiting response... HTTP request sent, awaiting response... HTTP request sent, awaiting response... HTTP request sent, awaiting response... 403 Forbidden
2026-02-26 13:39:22 ERROR 403: Forbidden.

403 Forbidden
2026-02-26 13:39:22 ERROR 403: Forbidden.

403 Forbidden
2026-02-26 13:39:22 ERROR 403: Forbidden.

ERROR:__main__:Corpus download for fuzz_advanced_api failed: Command '['wget', 'https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_advanced_api/public.zip', '-O', 'build/corpus/libwebp/fuzz_advanced_api.zip']' returned non-zero exit status 8..
ERROR:__main__:Corpus download for fuzz_animation_api failed: Command '['wget', 'https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_animation_api/public.zip', '-O', 'build/corpus/libwebp/fuzz_animation_api.zip']' returned non-zero exit status 8..
ERROR:__main__:Corpus download for fuzz_simple_api failed: Command '['wget', 'https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_simple_api/public.zip', '-O', 'build/corpus/libwebp/fuzz_simple_api.zip']' returned non-zero exit status 8..
403 Forbidden
403 Forbidden
403 Forbidden
2026-02-26 13:39:22 ERROR 403: Forbidden.

2026-02-26 13:39:22 ERROR 403: Forbidden.
2026-02-26 13:39:22 ERROR 403: Forbidden.

ERROR:__main__:Corpus download for fuzz_webp_enc_dec failed: Command '['wget', 'https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_webp_enc_dec/public.zip', '-O', 'build/corpus/libwebp/fuzz_webp_enc_dec.zip']' returned non-zero exit status 8..
ERROR:__main__:Corpus download for fuzz_webp_animencoder failed: Command '['wget', 'https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_webp_animencoder/public.zip', '-O', 'build/corpus/libwebp/fuzz_webp_animencoder.zip']' returned non-zero exit status 8..
ERROR:__main__:Corpus download for fuzz_demux_api failed: Command '['wget', 'https://storage.googleapis.com/libwebp-backup.clusterfuzz-external.appspot.com/corpus/libFuzzer/libwebp_fuzz_demux_api/public.zip', '-O', 'build/corpus/libwebp/fuzz_demux_api.zip']' returned non-zero exit status 8..
```

I also noticed on the [fuzz introspector page](https://introspector.oss-fuzz.com/project-profile?project=libwebp) all of the targets have `@` in their name which seems to be a FuzzTest autogenerated name. This also seems to be breaking the introspector report, e.g. https://storage.googleapis.com/oss-fuzz-introspector/libwebp/inspector-report/20260224/fuzz_report.html

Start with infra/helper.py and reproduce the public libwebp corpus download command, then inspect the generated target URLs and the linked Fuzz Introspector report. Compare the target names containing @ with the 403 responses and report failures. Done means identifying whether these are related and defining a verified correction or documenting the separate causes.

Written by the indexing model from the issue text.