fix(auth): address review — proxy CIMD discovery fetch, validate metadata path

Two findings from review:

- `ensureCimdClientRegistration` ran protected-resource discovery on the
  global fetch while its AS-metadata leg used the configured one. On web
  that is `createRemoteFetch`, which proxies through the backend to sidestep
  CORS — so the discovery this PR routes to a challenge-advertised URL would
  fail in the browser, be swallowed by the catch, and leave CIMD probing the
  wrong authorization server. Forward `params.fetchFn`, and assert in the
  test that nothing reaches the global.

- `oauth.resourceMetadataPath` accepted `//other-host/doc`. Resolved against
  the request base that re-points the origin in the advertised
  `resource_metadata` while Express still registers the route locally, so the
  server would advertise a document it does not serve. `/\\host` does the
  same, since the URL parser folds a backslash into a slash for special
  schemes — a leading-slash check misses both. Validate origin-relativity
  with a shared predicate, used by the JSON-config validator and again at
  server setup for a programmatically built ServerConfig.

Signed-off-by: cliffhall <cliff@futurescale.com>

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

@@ -105,24 +105,21 @@ describe("ensureCimdClientRegistration", () => {
     const metadataUrl = new URL(
       "http://127.0.0.1:9999/custom/protected-resource",
     );
-    // `ensureCimdClientRegistration` runs protected-resource discovery through
-    // the SDK's default fetch, so the advertised URL is observable only on the
-    // global — the AS-metadata leg is the one that takes `fetchFn`.
-    const globalFetch = vi.fn(async (input: RequestInfo | URL) =>
-      String(input) === metadataUrl.href
-        ? new Response(
-            JSON.stringify({
-              resource: SERVER_URL,
-              authorization_servers: ["http://127.0.0.1:9999"],
-            }),
-          )
-        : new Response(null, { status: 404 }),
-    );
-    vi.stubGlobal("fetch", globalFetch);
-
-    const fetchFn = vi.fn(
-      async () =>
-        new Response(
+    // Both discovery legs must run through the configured fetch — on web that
+    // is the backend proxy, so a leg that fell back to the global would fail
+    // on CORS and be swallowed.
+    const fetchFn = vi.fn(async (input: RequestInfo | URL) => {
+      const url = String(input);
+      if (url === metadataUrl.href) {
+        return new Response(
+          JSON.stringify({
+            resource: SERVER_URL,
+            authorization_servers: ["http://127.0.0.1:9999"],
+          }),
+        );
+      }
+      if (url.includes("/.well-known/oauth-authorization-server")) {
+        return new Response(
           JSON.stringify({
             issuer: "http://127.0.0.1:9999",
             authorization_endpoint: "http://127.0.0.1:9999/oauth/authorize",
@@ -130,8 +127,14 @@ describe("ensureCimdClientRegistration", () => {
             response_types_supported: ["code"],
             client_id_metadata_document_supported: true,
           }),
-        ),
-    );
+        );
+      }
+      return new Response(null, { status: 404 });
+    });
+    // Nothing may reach the network directly; a call here means a leg fell
+    // back to the global fetch.
+    const globalFetch = vi.fn();
+    vi.stubGlobal("fetch", globalFetch);
 
     await ensureCimdClientRegistration({
       serverUrl: SERVER_URL,
@@ -140,10 +143,14 @@ describe("ensureCimdClientRegistration", () => {
       resourceMetadataUrl: metadataUrl,
     });
 
-    expect(globalFetch).toHaveBeenCalled();
-    for (const [input] of globalFetch.mock.calls) {
-      expect(String(input)).toBe(metadataUrl.href);
-    }
+    expect(globalFetch).not.toHaveBeenCalled();
+    const requested = fetchFn.mock.calls.map(([input]) => String(input));
+    expect(requested).toContain(metadataUrl.href);
+    expect(
+      requested.filter((url) =>
+        url.includes("/.well-known/oauth-protected-resource"),
+      ),
+    ).toEqual([]);
     expect(storage.saveClientInformation).toHaveBeenCalledWith(
       SERVER_URL,
       { client_id: METADATA_URL },

clients/web/src/test/integration/mcp/oauth-resource-metadata-challenge.test.ts

@@ -17,6 +17,7 @@ import {
   TestServerHttp,
   getDefaultServerConfig,
   createOAuthTestServerConfig,
+  isOriginRelativePath,
   loadConfig,
   resolveConfig,
   waitForOAuthWellKnown,
@@ -103,4 +104,20 @@ describe("OAuth challenge resource_metadata (RFC 9728)", () => {
       resourceMetadataPath: METADATA_PATH,
     });
   });
+
+  it.each([
+    ["/custom/protected-resource", true],
+    ["/a", true],
+    // Both re-point the origin when resolved against the request base while
+    // Express still registers the route locally, so the server would advertise
+    // a document it does not serve (Copilot).
+    ["//other-host/doc", false],
+    ["/\\other-host/doc", false],
+    ["https://other-host/doc", false],
+    ["custom/protected-resource", false],
+    ["/has a space", false],
+    ["", false],
+  ])("origin-relative path check: %j -> %s", (value, expected) => {
+    expect(isOriginRelativePath(value)).toBe(expected);
+  });
 });