Treat inactive download contexts as cancellation, delete the partial file, and
rethrow the cancellation so the pipeline does not retain an incomplete payload.
Also remove orphaned .dsl/.dz files left behind by cancelled downloads.
Run the partial/orphan deletions on NonCancellable: the cleanup is invoked from
already-cancelled coroutines, where withContext(Dispatchers.IO) aborts before
executing the block and silently leaves the files behind.
OneWay
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