Add user agent to HTMLProofer

[samford]

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The brew documentation tests use HTMLProofer to check for broken links in generated documentation HTML and this is causing all brew PRs to fail CI because one of the linked pages will return a JavaScript challenge page if the request uses a user agent that the server doesn't like (or no user agent at all). The default HTMLProofer user agent is "Mozilla/5.0 (compatible; HTML Proofer/5.2.1; +https://github.com/gjtorikian/html-proofer)" and, for whatever reason, the Mozilla user agent triggers the upstream server's scraper safeguards.

This updates docs/Rakefile to use a generic "HTMLProofer/5.2.1" user agent for these requests, which addresses this particular failure. I've set this up to conditionally prepend HOMEBREW_USER_AGENT if it's available in the environment, though it's not currently present in this context, so I can drop that if it's not wanted/needed.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates the documentation link-check task to send a custom User-Agent with HTMLProofer requests, aiming to avoid CI failures caused by servers rejecting the default request profile.

Changes:

• Refactors the HTMLProofer invocation to build options in an args hash first.
• Reads HOMEBREW_USER_AGENT from the environment during the docs test task.
• Adds Typhoeus request headers so HTMLProofer requests send a User-Agent.

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[samford]

Ahh yeah, HOMEBREW_USER_AGENT isn't available in this context. Let me tinker with this some more.

[MikeMcQuaid]

Thanks! LGTM when 🟢. Merge sooner rather than later to unblock CI 🙇🏻