test: cover issue 8 forwarder residuals

[mrnicholasbcarter-code]

Summary

• add residual forwarder tests for 504 retry, retry exhaustion, invalid Retry-After fallback, client disconnect abort, mixed/partial SSE, and onError
• fix retry exhaustion so final retryable upstream response returns the upstream status/body instead of silently exiting

Verification

• npm run build
• npm test -- --runInBand
• npm run typecheck
• npm run format:check
• git diff --check

Closes #8