Repair Happy Eyeballs Race Bugs

Our Go HTTP client uses a custom Happy Eyeballs dialer. IPv6 starts first and IPv4 starts after the configured delay. When both connects complete at the same barrier, both goroutines can send a socket to the result channel; the second socket leaks, and during cancellation the cleanup path occasionally closes the actual winner. Repair with a fake resolver, fake conns, barriers, and a fake clock. Preserve address order, delay, timeout, TLS, pool, and retry settings; do not dial the network.

Dual-stack race found. IPv6 and IPv4 can both complete before the shared promise is marked, and the late callback can replace the selected socket while leaving the other open. The repair makes winner election generation-aware and atomic, closes every loser exactly once, preserves attempt errors, and validates simultaneous success, cancellation, timeout, and all-failure paths with fake sockets.