fix: enforce the OAuth deadline across the proxy hop (#2319 round 5)

Two findings, both real.

The race bounded only the caller's promise on the browser path.
`createRemoteFetch` dropped the signal, so an abort — including the
deadline's own — left the POST to /api/fetch in flight; the backend
never saw its request cancelled and its outbound fetch ran on with
neither signal nor timeout, holding a handler and an authorization-server
socket per timed-out attempt. `createRemoteFetch` now forwards the
caller's signal (init.signal when present and not undefined, else a
Request's own), and the /api/fetch route composes `c.req.raw.signal`
with a server-side deadline matching the client budget, cleared in
`finally` so a streaming response is not severed.

A probe's own timeout was swallowed by `withRfc8414OidcCompat`, which
returned the preceding 404 and discarded the endpoint name the deadline
exists to provide. A deadline the Inspector imposed is its own error,
not the server's answer, so it escapes now; every other probe failure
still falls back as documented. Nothing regresses: every candidate the
loop probes is one the SDK's own buildDiscoveryUrls emits, so a stall
there is one the SDK would have hit itself, unbounded.

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>

clients/web/src/test/core/auth/requestTimeout.test.ts

@@ -373,32 +373,25 @@ describe("withOAuthRequestTimeout", () => {
       expect(err.url).toBe(RFC8414);
     });
 
-    it("innermost, each leg is bounded on its own and the probe is abandoned", async () => {
+    it("innermost, the probe's own timeout escapes and names the probe", async () => {
       vi.useFakeTimers();
       const inner = stallingProbeFetch();
       const wrapped = withRfc8414OidcCompat(
         withOAuthRequestTimeout(inner, 1000),
       );
 
-      const settled = wrapped(RFC8414).then(
-        (response) => response,
-        (err: unknown) => err,
-      );
+      const assertion = wrapped(RFC8414).catch((err: unknown) => err);
       // The probe is timed from when it starts, on a budget of its own — not on
       // whatever an outer one had left after the RFC 8414 leg.
       await vi.advanceTimersByTimeAsync(1000);
 
-      const result = await settled;
-      // The compat wrapper is deliberately inert when a probe *throws* — it
-      // hands the original response back and lets discovery run its normal
-      // course — so a timed-out probe surfaces as the original 404 rather than
-      // as an error, and the second candidate is not tried. What innermost buys
-      // is that the probe is bounded at all, and that a stall in it can never be
-      // attributed to the RFC 8414 request the caller actually made.
-      expect(result).toBeInstanceOf(Response);
-      expect((result as Response).status).toBe(404);
+      const err = (await assertion) as OAuthRequestTimeoutError;
+      // The compat wrapper is inert on an ordinary probe failure, but a deadline
+      // the Inspector imposed escapes it, so the caller is told which endpoint
+      // stalled rather than being handed the preceding 404.
+      expect(err).toBeInstanceOf(OAuthRequestTimeoutError);
+      expect(err.url).toBe(PROBES[0]);
       expect(inner).toHaveBeenCalledTimes(2);
-      expect(String(inner.mock.calls[1][0])).toBe(PROBES[0]);
     });
   });
 

clients/web/src/test/core/mcp/remote/createRemoteFetch.test.ts

@@ -168,4 +168,77 @@ describe("createRemoteFetch", () => {
     expect(res.headers.get("x-test")).toBe("yes");
     expect(await res.text()).toBe("hello");
   });
+
+  describe("cancellation forwarding (#2319)", () => {
+    // Without this the caller's abort settled only its own promise: the POST
+    // stayed in flight, the backend never saw its request cancelled, and its
+    // outbound fetch to the authorization server ran on detached.
+    function capture() {
+      const fetchFn = vi
+        .fn<typeof fetch>()
+        .mockResolvedValue(ok(standardRemoteBody));
+      const remoteFetch = createRemoteFetch({
+        baseUrl: "http://remote.example",
+        fetchFn: fetchFn as unknown as typeof fetch,
+      });
+      return { fetchFn, remoteFetch };
+    }
+
+    function signalOf(fetchFn: ReturnType<typeof vi.fn<typeof fetch>>) {
+      return (fetchFn.mock.calls[0][1] as RequestInit).signal;
+    }
+
+    it("forwards init.signal onto the proxy hop", async () => {
+      const { fetchFn, remoteFetch } = capture();
+      const caller = new AbortController();
+
+      await remoteFetch("http://upstream.example/", { signal: caller.signal });
+
+      expect(signalOf(fetchFn)).toBe(caller.signal);
+    });
+
+    it("forwards a Request's own signal when init has none", async () => {
+      const { fetchFn, remoteFetch } = capture();
+      const caller = new AbortController();
+
+      await remoteFetch(
+        new Request("http://upstream.example/", { signal: caller.signal }),
+      );
+
+      expect(signalOf(fetchFn)).toBe(caller.signal);
+    });
+
+    it("treats an undefined init.signal as absent, keeping the Request's", async () => {
+      // WebIDL dictionary conversion: a member present as `undefined` is absent.
+      const { fetchFn, remoteFetch } = capture();
+      const caller = new AbortController();
+
+      await remoteFetch(
+        new Request("http://upstream.example/", { signal: caller.signal }),
+        { signal: undefined },
+      );
+
+      expect(signalOf(fetchFn)).toBe(caller.signal);
+    });
+
+    it("honours an explicit null init.signal as no signal", async () => {
+      const { fetchFn, remoteFetch } = capture();
+      const caller = new AbortController();
+
+      await remoteFetch(
+        new Request("http://upstream.example/", { signal: caller.signal }),
+        { signal: null },
+      );
+
+      expect(signalOf(fetchFn)).toBeUndefined();
+    });
+
+    it("sends no signal when the caller supplied none", async () => {
+      const { fetchFn, remoteFetch } = capture();
+
+      await remoteFetch("http://upstream.example/");
+
+      expect(signalOf(fetchFn)).toBeUndefined();
+    });
+  });
 });