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>

test-servers/src/composable-test-server.ts

@@ -573,6 +573,19 @@ export interface ServerConfig {
      */
     resource?: string;
 
+    /**
+     * Serve the RFC 9728 protected-resource metadata document from this
+     * non-default path *instead of* `/.well-known/oauth-protected-resource`,
+     * and advertise it via `WWW-Authenticate: Bearer resource_metadata="…"`
+     * on every 401.
+     *
+     * The default well-known routes are deliberately left unserved, so a
+     * client that ignores the advertised URL cannot discover the document at
+     * all — which is what makes this a regression test for #2071 rather than
+     * a path that merely happens to work.
+     */
+    resourceMetadataPath?: string;
+
     /**
      * OAuth authorization server issuer URL (combined mode AS metadata).
      * If not provided, defaults to the test server's base URL.