Fetch in parallel with brew bundle

[MikeMcQuaid]

Run brew fetch before brew install

This will parallelise fetching of formulae and casks in brew bundle

Note dependencies are not properly handled yet but that could be a future extension if someone cares enough.

Fixes #21140

[Copilot]

Pull request overview

This PR implements parallel fetching of formulae and casks in brew bundle by running brew fetch before brew install to improve performance. The implementation collects all packages that need to be installed or upgraded, fetches them in a single parallel operation, and then proceeds with sequential installation.

Key Changes

• Added prefetching logic that identifies formulae and casks needing installation/upgrade before the installation phase
• Introduced installable_or_upgradable? method in CaskInstaller to determine which casks should be fetched
• Modified installer workflow to fetch all applicable packages in parallel before sequential installation

Reviewed changes

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

File	Description
Library/Homebrew/bundle/installer.rb	Refactored to collect installable entries, added fetchable_formulae_and_casks method to identify packages for prefetching, and added fetch step before installation loop
Library/Homebrew/bundle/cask_installer.rb	Added installable_or_upgradable? helper method to determine if a cask needs fetching/installation
Library/Homebrew/test/bundle/installer_spec.rb	New test file covering prefetch behavior for both formulae and casks, including edge case where nothing needs fetching
Library/Homebrew/test/bundle/commands/install_spec.rb	Updated test mocks to accommodate new fetch calls and removed outdated expectation that system would never be called

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[bevanjkay]

I have completed some limited testing locally and can confirm it works for installations and upgrades. This is a great improvement!