Reset default download queue between specs

[MikeMcQuaid]

• Homebrew.default_download_queue memoizes its queue on the Homebrew module, so an example stubbing Homebrew::DownloadQueue.new at first use leaked an RSpec double into later examples and the at_exit shutdown hook, randomly crashing test runs with RSpec::Mocks::OutsideOfExampleError after every example passed.
• Drop the memoized queue after every example instead; calling shutdown in the around hook's ensure would itself run outside the rspec-mocks lifecycle.
• Add an ordered regression spec covering the leak.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Fable 5 Max with local review and testing.

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[Copilot]

Pull request overview

This PR fixes a test-suite flake caused by Homebrew.default_download_queue memoising a queue object across examples: if a spec stubs DownloadQueue.new on first use, the memoised value can remain an RSpec double and later trigger RSpec::Mocks::OutsideOfExampleError via the at_exit shutdown hook.

Changes:

• Add Homebrew.reset_default_download_queue to clear the memoised default queue between examples.
• Reset the memoised default download queue in spec_helper.rb after each example.
• Add an ordered regression spec to ensure a stubbed queue from one example does not leak into the next.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/test/spec_helper.rb	Resets the memoised default download queue after each example to prevent RSpec double leakage.
Library/Homebrew/test/download_queue_spec.rb	Adds an ordered regression spec covering the memoisation leak across examples.
Library/Homebrew/download_queue.rb	Introduces a reset helper for the memoised default download queue.

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