manager: stop and clean cancelled downloads

[Zhanfg]

Summary

Improve the Manager download lifecycle so cancelled or failed downloads do not
continue writing data or leave partial files in the Downloads directory.

Previously, cancelling a download cancelled the coroutine Job, but the blocking
response read loop did not explicitly observe coroutine cancellation. A task
could therefore continue processing data and potentially reach the completion
path after cancellation.

Failed or cancelled transfers could also leave partially downloaded files
behind. Subsequent downloads with the same filename would then be renamed,
accumulating stale partial files.

Changes

• check coroutine cancellation while streaming response data
• prevent a cancelled download from reaching markCompleted()
• remove partially written files after cancellation
• remove partially written files after download failures
• reserve the destination filename atomically with createNewFile()
• avoid races between simultaneous downloads choosing the same target name
• handle target-directory and target-file reservation failures through the
  normal failure/cleanup lifecycle
• cancel the active OkHttp Call together with the coroutine Job, so cancellation can interrupt a blocking network read
• preserve the Cancelled state when OkHttp surfaces call cancellation as an IOException

Testing

• clean assembleRelease -PIS_PR_BUILD=true passed before the final rebase
• rebased onto current main (af62466b70c0ebf8f9340e3a9bf0a487d804ca66)
• post-rebase diff contains one clean commit touching only DownloadManager.kt and DownloadService.kt
• fresh upstream Build Manager CI is awaiting upstream approval

[Copilot]

Pull request overview

Improves Manager download cancellation, cleanup, and destination reservation.

Changes:

• Cancels active OkHttp calls and observes coroutine cancellation.
• Deletes partial files on cancellation or failure.
• Atomically reserves unique destination files.

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