common : drop the partials of a removed sidecar
Removing a sidecar alone left the .downloadInProgress files of the sidecars a
download had planned in blobs/. The removal purges them through the same plan
the download follows, and the test covers a finished draft beside an unfinished
one, whose partial goes while the model keeps its files.
Assisted-by: DeepSeek-V4.1-Flash
