Bridges LlmEngine::extract_content_tags to the frontend with the same
spawn_blocking + PowerAssertion guard the cleanup_text command uses.
Returns a ContentTags object serialised to camelCase JSON. Errors
surface as readable strings so the frontend toast shows actionable
text on the rare grammar-bypass path.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>