feat(api): expose registered extension filters on /experiment/models (#14797)

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.

Co-authored-by: guill <jacob.e.segal@gmail.com>

openapi.yaml

@@ -775,6 +775,14 @@ components:
         ModelFolder:
             description: Represents a folder containing models
             properties:
+                extensions:
+                    description: The folder's registered file-extension allowlist. An empty array means the folder accepts any extension (match-all).
+                    example:
+                        - .ckpt
+                        - .safetensors
+                    items:
+                        type: string
+                    type: array
                 folders:
                     description: List of paths where models of this type are stored
                     example: