Fix cask_sparkle_min_os audit options handling

[samford]

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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.

I used Claude Code (Opus 5, high) to identify the source of the issue. I manually implemented a fix (confirming it with before/after runs of brew audit --online izip) and had Claude Code review the changes. Afterward, I used Claude Code to implement tests to bring coverage for the method up to 100% (for lines and branches). I reviewed the tests, cleaned them up, and confirmed that they achieved the desired coverage by targeting the various states that I had identified.

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The cask_sparkle_min_os audit calls livecheck's page_content method directly but it doesn't pass the livecheck block options, so the audit won't work as expected in that scenario. For example, we recently hit the user agent deprecation logic in a version bump PR for the izip cask (Homebrew/homebrew-cask#288419) but that livecheck block contains a user_agent: :browser option, so that shouldn't be possible. I traced it back to this specific audit and confirmed that passing the livecheck block options fixes the issue. I checked other usage of page_content and page_headers and existing calls pass options, as we would expect.

Besides that, I've added a guard to ensure that livecheck.url is set, as this check relies on a livecheck block URL being present. This is a relatively safe assumption but better safe than sorry.

This also adds tests to expand coverage for the cask_sparkle_min_os audit to 100% for lines and branches. To achieve this without using #send, it was necessary to make the method public (following recent guidance around this). The only functional change in the method is to use #fetch in the normalize_min_os argument, as we're already guarding against items.blank?, so we couldn't exercise the nil path for items[0].

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This bug can be replicated by running brew audit --online izip, which fails on the main branch (with the user agent deprecation error) and passes on this PR branch.

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[MikeMcQuaid]

Thanks!