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/backends/base.py

@@ -48,11 +48,18 @@ def is_model_cached(
         if not repo_cache.exists():
             return False
 
-        # Incomplete blobs mean a download is still in progress
+        # An .incomplete blob means a download is still in progress -- unless
+        # a completed blob with the same hash already sits next to it, which
+        # happens when a retried/concurrent download leaves a stale .incomplete
+        # behind after the real transfer already finished. Only orphaned
+        # .incomplete files (no matching completed blob) count as "in progress".
         blobs_dir = repo_cache / "blobs"
-        if blobs_dir.exists() and any(blobs_dir.glob("*.incomplete")):
-            logger.debug(f"Found .incomplete files for {hf_repo}")
-            return False
+        if blobs_dir.exists():
+            for incomplete in blobs_dir.glob("*.incomplete"):
+                completed = incomplete.with_name(incomplete.name.removesuffix(".incomplete"))
+                if not completed.exists():
+                    logger.debug(f"Found in-progress .incomplete file for {hf_repo}")
+                    return False
 
         snapshots_dir = repo_cache / "snapshots"
         if not snapshots_dir.exists():