Add previously covered autolink test cases that were dropped during the
goldmark migration (hosts without dots, https variants, single-digit
issues, cross-repo issues). Add new test suites for link rewriting with
both path-only and absolute URL prefixes, and for HTML passthrough
behavior confirming raw HTML is stripped without WithUnsafe.
Sanitize RawMarkdown output in the server notice banner to prevent
potential XSS, since it was the only call site not passing through
SanitizeBytes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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