## Summary
- Disable HTTP keep-alive when a proxy returns a 407 (Proxy
Authentication Required) status code
- This prevents subsequent requests from trying to reuse a connection
that the proxy server has closed
- Refactored proxy tests to use `describe.concurrent` and async
`Bun.spawn` patterns
## Test plan
- [x] Added test `simultaneous proxy auth failures should not hang` that
verifies multiple concurrent requests with invalid proxy credentials
complete without hanging
- [x] Existing proxy tests pass
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