Use cask dependencies for platform support

[MikeMcQuaid]

• Keep sha256 responsible only for selecting a checksum.
• Use OS and architecture depends_on declarations as the source of truth for supported platforms.
• Reject missing checksums before bump-cask-pr attempts a download.

Fixes #23494

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[Copilot]

Pull request overview

This PR adjusts cask platform support handling so depends_on (OS/arch) declarations are treated as the source of truth for supported platforms, while sha256 only selects an appropriate checksum. It also updates bump-cask-pr to avoid attempting downloads for unsupported platforms and to fail fast when a checksum is missing.

Changes:

• Teach bump-cask-pr to skip OSes not supported by the cask’s OS depends_on, and to raise early when the computed checksum is missing.
• Simplify the cask sha256 DSL to stop raising when the selected checksum is nil.
• Update/add specs to enforce the new expectations (missing checksums require explicit depends_on constraints).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/dev-cmd/bump-cask-pr.rb	Skips unsupported OS simulations via supports_macos?/supports_linux? and raises a targeted error when sha256 resolves to nil.
Library/Homebrew/cask/dsl.rb	Removes real-system raising behavior for nil checksum selection, leaving sha256 as a selector only.
Library/Homebrew/test/dev-cmd/bump-cask-pr_spec.rb	Updates fixtures to include OS depends_on and adds a regression spec ensuring missing checksums are rejected before download.
Library/Homebrew/test/cask/dsl_spec.rb	Updates checksum behavior expectations and asserts arch constraints can be used to declare unsupported architectures.

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