The existing regex only matched the old format where account_uuid is
empty (account__session_). Real Claude Code clients and newer sub2api
generated user_ids use account_{uuid}_session_ which was silently
skipped, causing the original metadata.user_id to leak to upstream
when User-Agent is rewritten by an intermediate gateway.
Closes #766
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>