Each folder in the listing now carries its registered extension
allowlist verbatim; an empty array means the folder accepts any
extension (match-all), mirroring filter_files_extensions semantics.
Gives consumers the filtering rule itself rather than just its output:
/models/{folder} lists files by the per-folder rule but the rule is not
exposed anywhere, and /experiment/models/{folder} filters everything by
the global supported_pt_extensions regardless of registration.
Presentation-level filtering of match-all folders (e.g. hiding
README/config noise that repository-downloading custom nodes leave in
model directories) is deliberately left to the consumer.
