sorbet: Bump more Cask tests to higher typed levels

[issyl0]

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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 Code CLI, Claude Opus 5 (High reasoning, Ultracode was too intense 🤯), with human prompting, review and tweaks.

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• Part of my ongoing quest to bump all the test files to at least Sorbet typed: true, because that's a thing we can do.
• In a 🥞 gh stack because I wanted to try them out on a real chain of changes.
• See the commit message body for the full description of the changes.

[Copilot]

Pull request overview

This PR advances Sorbet adoption in Homebrew’s Cask area by raising the typed levels of additional Cask-related specs and extending the Tapioca-generated RBI for Cask::DSL to cover architecture-scoped DSL helpers.

Changes:

• Bump selected Cask-related specs from # typed: false to # typed: true and adjust a couple of let declarations to satisfy typechecking.
• Extend the Cask::DSL Tapioca compiler to generate RBI entries for on_arm/on_intel.
• Update the autogenerated Cask::DSL RBI to include on_arm/on_intel signatures.

Show a summary per file

File	Description
Library/Homebrew/test/dev-cmd/bump-cask-pr_spec.rb	Raises the spec’s Sorbet typed level to improve static checking coverage.
Library/Homebrew/test/cask/download_spec.rb	Raises the spec’s Sorbet typed level and tweaks let scoping for typechecking.
Library/Homebrew/sorbet/tapioca/compilers/cask/dsl.rb	Generates RBI method signatures for Cask::DSL#on_arm/#on_intel.
Library/Homebrew/sorbet/rbi/dsl/cask/dsl.rbi	Adds the generated on_arm/on_intel methods to the Cask DSL RBI.

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• Files reviewed: 3/4 changed files
• Comments generated: 2
• Review effort level: Lite