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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>