Omit platform for all bottles

[MikeMcQuaid]

• Avoid labelling :all OCI descriptors as platform-specific
• Strip platform build fields from :all tab annotations
• Cover the brew bottle --merge JSON that passes all onward
• Cover GitHub Packages descriptors for all bottles

Closes #17395

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[Copilot]

Pull request overview

This pull request updates Homebrew’s GitHub Packages (OCI) bottle publishing to treat :all bottles as platform-agnostic by omitting OCI descriptor platform data and removing platform build fields from the embedded sh.brew.tab annotation. It also adds test coverage to ensure brew bottle --merge produces an all bottle JSON artifact and that GitHub Packages output matches the intended OCI behavior.

Changes:

• Omit the OCI image index manifest platform field when uploading :all bottles.
• Strip arch/built_on from the sh.brew.tab annotation for :all bottles.
• Add RSpec coverage for both brew bottle --merge producing an all JSON and GitHub Packages’ all descriptor behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/github_packages.rb	Drops platform for :all descriptors and removes platform build fields from the sh.brew.tab annotation; widens Sorbet types for schema validation/writers to allow symbol keys.
Library/Homebrew/test/github_packages_spec.rb	New spec asserting :all descriptors omit platform and sh.brew.tab omits arch/built_on, while platform-specific bottles retain them.
Library/Homebrew/test/dev-cmd/bottle_spec.rb	Adds coverage that brew bottle --merge writes an all bottle JSON and copies the all bottle file as expected.

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