fix(cache): don't treat orphaned .incomplete blobs as an in-progress download

is_model_cached() marked a model as not-cached whenever any .incomplete
blob existed in its cache dir, even when a completed blob with the same
hash already sat next to it. A retried/concurrent download can leave
this orphan behind after the real transfer already finished, which made
the model appear perpetually "downloading" and re-trigger a full
re-download on every load.

Only .incomplete files with no matching completed blob now count as a
genuinely in-progress download.

backend/tests/test_is_model_cached.py

@@ -0,0 +1,74 @@
+"""
+Unit tests for ``is_model_cached``'s handling of stale ``.incomplete`` blobs.
+
+A retried or concurrent download can leave an orphaned ``.incomplete`` file
+next to its now-completed counterpart (same blob hash, no suffix). Only a
+genuinely in-progress download -- an ``.incomplete`` with no completed blob
+alongside it -- should mark the model as not cached.
+
+``is_model_cached`` is extracted and exec'd standalone (instead of importing
+``backend.backends.base``) so this test doesn't pull in the module's sibling
+imports (audio/progress/hf_progress/tasks), which in turn require the full
+ML stack (torch/transformers/librosa/fastapi/...) this pure filesystem check
+never touches.
+"""
+
+import ast
+import logging
+from pathlib import Path
+from typing import Optional
+
+_SOURCE = (Path(__file__).parent.parent / "backends" / "base.py").read_text()
+_MODULE = ast.parse(_SOURCE)
+_FUNC_SRC = next(
+    ast.get_source_segment(_SOURCE, node)
+    for node in _MODULE.body
+    if isinstance(node, ast.FunctionDef) and node.name == "is_model_cached"
+)
+
+_namespace = {
+    "Path": Path,
+    "Optional": Optional,
+    "logger": logging.getLogger("test_is_model_cached"),
+}
+exec(_FUNC_SRC, _namespace)  # noqa: S102
+is_model_cached = _namespace["is_model_cached"]
+
+
+def _make_repo_cache(tmp_path, repo="org/model"):
+    repo_dir = tmp_path / ("models--" + repo.replace("/", "--"))
+    blobs_dir = repo_dir / "blobs"
+    snapshots_dir = repo_dir / "snapshots" / "abc123"
+    blobs_dir.mkdir(parents=True)
+    snapshots_dir.mkdir(parents=True)
+    return repo_dir, blobs_dir, snapshots_dir
+
+
+def test_orphaned_incomplete_blob_does_not_block_cache_hit(tmp_path, monkeypatch):
+    import huggingface_hub.constants as hf_constants
+
+    monkeypatch.setattr(hf_constants, "HF_HUB_CACHE", str(tmp_path))
+
+    repo = "org/model"
+    _, blobs_dir, snapshots_dir = _make_repo_cache(tmp_path, repo)
+
+    completed_blob = blobs_dir / "deadbeef"
+    completed_blob.write_bytes(b"weights")
+    (blobs_dir / "deadbeef.incomplete").write_bytes(b"stale partial")
+    (snapshots_dir / "model.safetensors").symlink_to(completed_blob)
+
+    assert is_model_cached(repo) is True
+
+
+def test_genuinely_in_progress_download_is_not_cached(tmp_path, monkeypatch):
+    import huggingface_hub.constants as hf_constants
+
+    monkeypatch.setattr(hf_constants, "HF_HUB_CACHE", str(tmp_path))
+
+    repo = "org/model"
+    _, blobs_dir, snapshots_dir = _make_repo_cache(tmp_path, repo)
+
+    (blobs_dir / "feedface.incomplete").write_bytes(b"partial")
+    (snapshots_dir / "config.json").write_text("{}")
+
+    assert is_model_cached(repo) is False