fix: carry the proxy deadline's identity, correct the rounding note (#2319 round 9)

A deadline enforced by the backend could not arrive as an
OAuthRequestTimeoutError: /api/fetch serialized it as a 500 and
createRemoteFetch turned that into a plain Error, so every instanceof
check downstream was false — including the one that lets a stalled
probe escape withRfc8414OidcCompat. That matters most where no
client-side wrapper exists to have fired first: the discovery the SDK
runs from inside the transport. The route now answers 504 with a typed
marker (OAUTH_TIMEOUT_WIRE_CODE plus url and timeoutMs) and
createRemoteFetch reconstructs the error from it, guarded by a shape
check so an upstream's own JSON cannot forge one.

The route's deadline becomes a `fetchTimeoutMs` option defaulting to
the shared constant, so the backstop can be proven to fire in
milliseconds instead of thirty seconds of wall clock — and because a
very slow authorization server is a legitimate reason to want a longer
one.

The rounding rationale was wrong: setTimeout accepts a fractional delay
and truncates it (verified on Node 26); ERR_OUT_OF_RANGE is
AbortSignal.timeout's behaviour, which this wrapper never calls. The
rounding stays, for the reason that is actually true — the budget is
interpolated into the message and exposed as timeoutMs, and a
performance.now() budget would otherwise report 1000.4000000953674ms.

Also drops a sentence in the discarded-stream test that contradicted
the qualification above it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YFnM6RqB4nXqNysgUYmjmq
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/remote/createRemoteFetch.ts

@@ -21,6 +21,11 @@
  *     the web client against a strict modern server, same as from the CLI/TUI.
  */
 
+import {
+  isOAuthRequestTimeoutWire,
+  OAuthRequestTimeoutError,
+} from "../../auth/requestTimeout.js";
+
 export interface RemoteFetchOptions {
   /** Base URL of the remote server (e.g. http://localhost:3000) */
   baseUrl: string;
@@ -158,6 +163,22 @@ export function createRemoteFetch(options: RemoteFetchOptions): typeof fetch {
 
     if (!res.ok) {
       const text = await res.text();
+      // A deadline the backend enforced arrives as an ordinary error response;
+      // rebuild the typed error from its marker so the `instanceof` checks
+      // downstream — most importantly the one that lets a stalled probe escape
+      // `withRfc8414OidcCompat` — still see a timeout for what it is (#2319,
+      // Copilot). This is the only path on which such a timeout can reach the
+      // browser without a client-side wrapper having fired first: the discovery
+      // the SDK runs from inside the transport has no wrapper at all.
+      let parsed: unknown;
+      try {
+        parsed = JSON.parse(text);
+      } catch {
+        parsed = undefined;
+      }
+      if (isOAuthRequestTimeoutWire(parsed)) {
+        throw new OAuthRequestTimeoutError(parsed.url, parsed.timeoutMs);
+      }
       throw new Error(`Remote fetch failed (${res.status}): ${text}`);
     }