Harden developer guide redirect updates

[shai-almog]

Summary

• fall back to the newest stable Codename One release that actually contains developer-guide.pdf when the latest release is still uploading assets
• exclude draft and prerelease releases from the production fallback
• keep failing when no valid release asset exists at all
• add fixture-backed regression coverage and run it in website CI

Root cause

GitHub can expose a newly published release through the latest-release API before the developer guide workflow finishes attaching developer-guide.pdf. The website updater treated that temporary state as fatal, aborting unrelated website and Port Status deployments.

Impact

During the upload gap, the website now retains a valid developer-guide download by pointing at the newest earlier stable release with the asset. A later website build automatically advances the redirect after the new asset becomes available.

Validation

• passed fixture tests for a populated latest release
• passed fixture tests for the release-upload race and previous stable fallback
• verified prereleases are excluded
• verified a genuine absence still fails without modifying redirects
• passed Bash syntax checks and actionlint 1.7.12
• passed the local production Hugo build and Port Status validation
• validated the generated developer-guide redirect against current GitHub release metadata

[github-actions]

Cloudflare Preview

• URL: https://pr-5397-website-preview.codenameone.pages.dev
• Branch: pr-5397-website-preview