fix(proxy-retry): gate streamIdleTimeoutMs watchdog to retry modes (not off)

The watchdog (added in 02e295f) wraps response.body in a TransformStream
to bound idle on a hung upstream stream. But the interceptor
(server/interceptor.js) already reconstructs response.body via getReader()
+ a new ReadableStream for logging/live-streaming. Under CI's concurrent
load the watchdog's pipeThrough races that reconstruction and surfaces as
a spurious `fetch failed` → status 0 → 502 on a healthy streaming 200.

Gate the watchdog to retry modes only (serial/race/stagger), mirroring the
existing connectTimeoutMs off-exclusion. off mode is the legacy pass-through
path (no retry): the watchdog's value (bound idle on a hung stream) is
marginal there since the interceptor already observes the stream, and
gating it off restores byte-identical pre-watchdog streaming behavior.

Verified: proxy-server.test.js streaming-200 test passes 3/3 under
full test:cli flags (--test-force-exit, --test-timeout=120000);
proxy-retry-stream-idle.test.js still passes (hung-stream aborts in
serial mode, healthy stream does not).

Author: souloss

server/lib/proxy-retry.js

@@ -496,9 +496,16 @@ export async function executeRequest({ url, fetchOptions, retryConfig, ctx }) {
   // headers well past 10s — so with retry disabled we must not introduce a new
   // failure mode. The timeout applies only when a retry mode is active.
   const effectiveConnectTimeoutMs = cfg.mode === 'off' ? 0 : cfg.connectTimeoutMs;
-  // streamIdleTimeoutMs applies to streaming responses in ALL modes (including off):
-  // even with retry disabled, a hung streaming body must not pin sockets forever.
-  const effectiveStreamIdleMs = cfg.streamIdleTimeoutMs > 0 ? cfg.streamIdleTimeoutMs : 0;
+  // streamIdleTimeoutMs is gated to retry modes only (NOT off), mirroring the
+  // connectTimeoutMs off-exclusion above. The watchdog wraps response.body in a
+  // TransformStream, but the interceptor (server/interceptor.js) already
+  // reconstructs response.body via getReader() + a new ReadableStream for
+  // logging/live-streaming; under concurrent load the watchdog's pipeThrough
+  // races that reconstruction and surfaces as a spurious `fetch failed` →
+  // status 0 → 502. off mode is the legacy pass-through path (no retry), so the
+  // watchdog's value (bound idle on a hung stream) is marginal here and the
+  // interceptor already observes the stream — serial/race/stagger keep the guard.
+  const effectiveStreamIdleMs = cfg.mode === 'off' ? 0 : (cfg.streamIdleTimeoutMs > 0 ? cfg.streamIdleTimeoutMs : 0);
   const commonCtx = { dispatcher, connectTimeoutMs: effectiveConnectTimeoutMs, streamIdleTimeoutMs: effectiveStreamIdleMs };
 
   if (cfg.mode === 'off' || cfg.mode === 'serial') {