fix(runspec): support art://name/file URIs without version specifier

[andrewwhitecdw]

Problem

ArtifactFileSystem documents this URI form:

art://name/path/to/file  (implies latest)

But when no :version specifier is present, _parse_uri treats the entire path (including the file part) as the artifact name:

fs._parse_uri("art://my-dataset/train.json")
# -> ('my-dataset/train.json', None, '')   # name swallows the file path

Since the resulting name contains /, _resolve() routes it to the W&B full-path resolver, so opening a file from a local registry artifact fails (e.g. UsageError: No API key configured).

Fix

In the no-version branch, split the first segment off as the artifact name when it exists in the local registry; otherwise keep the whole path as the artifact reference (preserving W&B entity/project/name paths without a version).

fs._parse_uri("art://my-dataset/train.json")   # ('my-dataset', None, 'train.json')
fs._parse_uri("art://my-dataset:v1/train.json") # ('my-dataset', 1, 'train.json')   (unchanged)
fs._parse_uri("art://my-dataset")               # ('my-dataset', None, '')          (unchanged)
fs._parse_uri("art://entity/project/name")      # ('entity/project/name', None, '') (unchanged)

Testing

• Verified all forms above plus end-to-end fs._open("art://my-dataset/train.json") against a local fsspec registry
• pytest tests/nemo_runspec -k "filesystem or artifact or registry" — 34 passed
• Full tests/nemo_runspec — 186 passed, 3 pre-existing failures on main (unrelated data_mover/lepton tests)