Reject unsupported API casks earlier

[MikeMcQuaid]

• stop gimp on Linux before source reloads fail
• avoid queued downloads for unsupported API-backed casks
• cover fetch and enqueue_downloads with regression tests

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[Copilot]

Pull request overview

This PR changes cask installation flow to fail fast on unsupported API-backed casks (before attempting API source reloads or queueing downloads), with regression tests covering both fetch and download prefetching.

Changes:

• Run check_requirements before load_cask_from_source_api! in Cask::Installer#fetch.
• Run check_requirements at the start of Cask::Installer#enqueue_downloads to prevent queuing downloads for unsupported casks.
• Add installer specs ensuring API source download paths are not invoked when requirements are unsatisfied.

Reviewed changes

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

File	Description
Library/Homebrew/cask/installer.rb	Reorders/introduces requirement checks to reject unsupported API-backed casks earlier in fetch and enqueue_downloads.
Library/Homebrew/test/cask/installer_spec.rb	Adds regression tests asserting requirements are validated before any API source-download work in fetch and before prefetch enqueuing.

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