Fix flaky DownloadQueue interruption test

[MikeMcQuaid]

• Cancel staging when Interrupt arrives during fetch setup
• Synchronise the regression test with the vulnerable setup window

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[Copilot]

Pull request overview

This PR fixes a race window in Homebrew::DownloadQueue#fetch so that an Interrupt arriving during early fetch setup reliably cancels active worker threads, and updates the regression spec to synchronise precisely with that vulnerable window to eliminate flakiness.

Changes:

• Add a top-level rescue Exception in DownloadQueue#fetch to ensure cancel runs even if Interrupt occurs before the parallel fetch loop is fully established.
• Adjust the existing TTY-update rescue path to focus on restoring terminal state, relying on the new outer rescue for cancellation.
• Stabilise the “interrupts queued staging” spec by gating the interrupt on both “fetch setup started” and “staging started”.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/download_queue.rb	Ensures cancellation runs on Interrupt even during early fetch setup, while keeping TTY restoration intact.
Library/Homebrew/test/download_queue_spec.rb	Synchronises the interrupt regression test with the fetch-setup timing window to reduce flakiness.

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