The default direct dispatcher pools keep-alive sockets for up to
fetchKeepAliveTimeoutMs (4s). Edges like nvidia / opencode-zen silently
close idle keep-alive sockets within that window, so the next request
reusing a pooled socket fails with UND_ERR_SOCKET ("other side closed") —
in bursts. proxyFetch retried once, but the retry reused the SAME pooled
dispatcher and could grab another stale socket, then fell through to native
fetch (which also pools) → the job sat in the rate-limit queue until the
30s timeout → 502 + circuit breaker open.
Add getRetryDispatcher() (no keep-alive, no pipelining, mirrors the proxy
dispatcher mitigation) and use it for the retry attempt so it opens a fresh
socket that can't be a dead pooled one. The first attempt still uses the
pooled dispatcher, preserving healthy keep-alive reuse.
Regression test (DI mock) asserts the retry uses getRetryDispatcher(), not
getDefaultDispatcher() (RED before, GREEN after).
Refs #4281 (describeFetchCause diagnostics). Closes#4252
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