simulate-macos: use newest supported version

[MikeMcQuaid]

• HOMEBREW_SIMULATE_MACOS_ON_LINUX=1 resolved the OS to the generic :macos, so SimulateSystem.current_tag built x86_64_macos / arm64_macos tags.
• The internal JSON API only publishes per-version files (e.g. packages.tahoe.jws.json), so fetching packages.x86_64_macos.jws.json returned 404 and broke any brew test-bot --only-tap-syntax run on Linux.
• Resolve to HOMEBREW_MACOS_NEWEST_SUPPORTED instead, matching what generate-internal-api and friends already do.

Fixes #22247

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Claude Code Opus 4.7 xhigh with local review and testing.

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[Copilot]

Pull request overview

This PR fixes HOMEBREW_SIMULATE_MACOS_ON_LINUX=1 so simulated macOS on Linux resolves to the newest supported versioned macOS symbol (e.g. :tahoe) rather than the generic :macos, preventing internal API requests from using a non-published tag like x86_64_macos and returning 404s.

Changes:

• Update Linux SimulateSystem.os to return MacOSVersion.new(HOMEBREW_MACOS_NEWEST_SUPPORTED).to_sym when simulating macOS.
• Update the corresponding RSpec expectation to match the new behavior.

Reviewed changes

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

File	Description
Library/Homebrew/extend/os/linux/simulate_system.rb	Uses the newest supported macOS symbol for HOMEBREW_SIMULATE_MACOS_ON_LINUX, ensuring generated tags match published internal API files.
Library/Homebrew/test/simulate_system_spec.rb	Adjusts the Linux simulation test to assert the newest supported macOS symbol instead of :macos.

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