upgrade: ignore failed shared prefetches

[MikeMcQuaid]

• keep DownloadQueue#fetch output and failure handling unchanged while recording whether a shared fetch failed
• stop brew upgrade reusing prefetched downloads after a failed shared fetch so later retries download again

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[Copilot]

Pull request overview

This PR fixes a brew upgrade edge case where a failed shared (combined) prefetch could cause subsequent upgrade logic to incorrectly treat downloads as reusable, preventing proper retry behavior. It does so by tracking whether a DownloadQueue#fetch run encountered any failures and disabling reuse of prefetched downloads when that happens.

Changes:

• Add DownloadQueue#fetch_failed state to record whether fetch encountered rejected/failed downloads.
• Update brew upgrade to avoid reusing prefetched downloads when the shared prefetch had any failures.
• Add/extend unit coverage for both the new fetch_failed behavior and the upgrade flow decision.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/download_queue.rb	Tracks whether a fetch had any failures via @fetch_failed and exposes it via fetch_failed.
Library/Homebrew/cmd/upgrade.rb	Disables use_prefetched / skip_prefetch when the shared download queue reports a failed fetch.
Library/Homebrew/test/download_queue_spec.rb	Adds a spec ensuring parallel-mode rejected downloads set fetch_failed and mark Homebrew failed.
Library/Homebrew/test/cmd/upgrade_spec.rb	Updates existing mocking for fetch_failed and adds coverage for not trusting failed shared prefetches.

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