Share cask upgrade download queues

[MikeMcQuaid]

• Avoid splitting cask source fetches from combined upgrade fetches
• Keep language-specific caskfiles on a prefetch queue only when needed
• Hide Downloading Cask files unless that queue will fetch new files

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[Copilot]

Pull request overview

This PR refactors cask prefetching so source-API caskfile downloads can share the same DownloadQueue used for combined formula/cask upgrade or install fetches, while keeping an early caskfile fetch only for language-specific casks that need it before resolving the final download URL.

Changes:

• Adds cask installer prefetch helpers and splits cask source download construction from enqueue/fetch side effects.
• Updates install/reinstall/upgrade paths to pass shared download queues into cask enqueueing.
• Adds/updates specs covering shared queues, language caskfile prefetches, cached caskfiles, and changed installer double interfaces.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Library/Homebrew/cask/installer.rb	Adds source caskfile prefetch handling and refactors prelude/enqueue flow.
Library/Homebrew/api/cask.rb	Introduces source_download_for factory used by both prefetch and existing source download paths.
Library/Homebrew/install.rb	Updates cask installer enqueueing to use a caller-provided shared queue and conditional caskfile prefetch.
Library/Homebrew/cmd/install.rb	Passes the shared install download queue into cask enqueueing.
Library/Homebrew/cmd/reinstall.rb	Passes the shared reinstall download queue into cask enqueueing.
Library/Homebrew/cmd/upgrade.rb	Passes the shared upgrade download queue into cask enqueueing.
Library/Homebrew/cask/upgrade.rb	Uses Homebrew::Install.enqueue_cask_installers for cask upgrade prefetching.
Library/Homebrew/cask/reinstall.rb	Uses Homebrew::Install.enqueue_cask_installers for cask reinstall prefetching.
Library/Homebrew/test/cask/installer_spec.rb	Adds coverage for language and non-language source-API cask prefetch behavior.
Library/Homebrew/test/cmd/upgrade_spec.rb	Adds coverage for shared queues and caskfile prefetch heading behavior.
Library/Homebrew/test/cmd/install_spec.rb	Updates installer doubles for the new enqueue contract.
Library/Homebrew/test/cask/upgrade_spec.rb	Updates installer doubles for the new enqueue contract.
Library/Homebrew/test/cask/reinstall_spec.rb	Updates installer doubles for the new enqueue contract.

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