Fix Japanese translation placeholders for PR contributor metadata and CustomContent

[qiancai]

Summary

• Preserve inline same-paragraph CustomContent pairs as atomic placeholders while translating their inner content.
• Restore standalone CustomContent indentation from the source Markdown so list-scoped blocks keep their original nesting.
• Restore the leading space before trailing GitHub issue links such as [#12277] when translation joins them to the preceding text.
• Fixes Google Translate unexpectedly injects trailing PR contributor metadata into Japanese release notes website-docs#687

Tests

• cd markdown-translator && node --test