fix(auth): pass the challenge's resource_metadata URL to SDK auth()

An RFC 9728 `WWW-Authenticate: Bearer resource_metadata="…"` challenge was
parsed and then dropped by the normalized `AuthChallenge`, so every
challenge-driven `mcpAuth()` call ran discovery without it and probed
locations derived from the MCP server URL instead. OAuth failed outright
against any server whose metadata document lives off the well-known path.

The SDK handles this correctly inside its own transports, but the Inspector
never reaches that path — it intercepts the 401 and drives `auth()` itself
to capture the authorization URL and run the step-up scope union.

Carry the advertised URL on `AuthChallenge` as a string (it crosses the web
client's remote-backend boundary as JSON), convert it at the OAuth boundary,
and pass it to the standard challenge leg, the forced step-up leg, and CIMD
pre-registration — which runs before `auth()` and did its own discovery. A
malformed value is ignored rather than surfaced, matching the SDK's parser.
The callback leg needs nothing: SDK `auth()` persists the URL in its
discovery state.

Adds an `oauth.resourceMetadataPath` test-server mode that moves the
document off the well-known path entirely, so the integration test proves
discovery genuinely fails without the fix.

Closes #2071

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

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

@@ -0,0 +1,106 @@
+/**
+ * RFC 9728 `resource_metadata` end-to-end, against a real OAuth test server
+ * whose protected-resource metadata document lives at a non-default path and
+ * whose well-known locations deliberately 404 (#2071).
+ *
+ * The unit tests prove the URL is parsed and handed to `mcpAuth()`. This file
+ * proves the two halves that only a real server can: that the challenge the
+ * server actually emits parses into `AuthChallenge.resourceMetadataUrl`, and
+ * that SDK discovery genuinely cannot find the document without it — so the
+ * plumbing is load-bearing rather than merely present.
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from "vitest";
+import { fileURLToPath } from "node:url";
+import { discoverOAuthProtectedResourceMetadata } from "@modelcontextprotocol/client";
+import {
+  TestServerHttp,
+  getDefaultServerConfig,
+  createOAuthTestServerConfig,
+  loadConfig,
+  resolveConfig,
+  waitForOAuthWellKnown,
+} from "@modelcontextprotocol/inspector-test-server";
+import {
+  challengeResourceMetadataUrl,
+  parseAuthChallengeFromResponse,
+} from "@inspector/core/auth/challenge.js";
+
+const METADATA_PATH = "/custom/protected-resource";
+
+describe("OAuth challenge resource_metadata (RFC 9728)", () => {
+  let mcpServer: TestServerHttp | null = null;
+  let serverUrl = "";
+
+  beforeAll(async () => {
+    mcpServer = new TestServerHttp({
+      ...getDefaultServerConfig(),
+      serverType: "streamable-http" as const,
+      ...createOAuthTestServerConfig({
+        requireAuth: true,
+        resourceMetadataPath: METADATA_PATH,
+      }),
+    });
+    const port = await mcpServer.start();
+    serverUrl = `http://localhost:${port}`;
+    await waitForOAuthWellKnown(serverUrl);
+  }, 30_000);
+
+  afterAll(async () => {
+    await mcpServer?.stop();
+    mcpServer = null;
+  }, 30_000);
+
+  it("advertises the non-default metadata URL on the 401 challenge", async () => {
+    const response = await fetch(`${serverUrl}/mcp`, { method: "POST" });
+
+    expect(response.status).toBe(401);
+    const challenge = parseAuthChallengeFromResponse(response);
+    expect(challenge?.resourceMetadataUrl).toBe(`${serverUrl}${METADATA_PATH}`);
+    expect(challengeResourceMetadataUrl(challenge!)?.href).toBe(
+      `${serverUrl}${METADATA_PATH}`,
+    );
+  });
+
+  it("serves the metadata document only from the advertised path", async () => {
+    const advertised = await fetch(`${serverUrl}${METADATA_PATH}`);
+    expect(advertised.status).toBe(200);
+    await expect(advertised.json()).resolves.toMatchObject({
+      authorization_servers: [serverUrl],
+    });
+
+    const wellKnown = await fetch(
+      `${serverUrl}/.well-known/oauth-protected-resource`,
+    );
+    expect(wellKnown.status).toBe(404);
+  });
+
+  it("SDK discovery resolves with the advertised URL and fails without it", async () => {
+    await expect(
+      discoverOAuthProtectedResourceMetadata(serverUrl, {
+        resourceMetadataUrl: new URL(`${serverUrl}${METADATA_PATH}`),
+      }),
+    ).resolves.toMatchObject({ authorization_servers: [serverUrl] });
+
+    // The pre-fix behavior: discovery derived from the MCP server URL alone.
+    await expect(
+      discoverOAuthProtectedResourceMetadata(serverUrl),
+    ).rejects.toThrow();
+  });
+
+  it("keeps the showcase config valid and carrying the custom path", () => {
+    const configPath = fileURLToPath(
+      new URL(
+        "../../../../../../test-servers/configs/oauth-custom-resource-metadata-http.json",
+        import.meta.url,
+      ),
+    );
+
+    // loadConfig validates as it parses, so this is the regression test for
+    // the checked-in showcase config as much as for the new field.
+    expect(resolveConfig(loadConfig(configPath)).oauth).toMatchObject({
+      enabled: true,
+      resourceMetadataPath: METADATA_PATH,
+    });
+  });
+});