feat(endorser): add client helper to check JWT via rateLimits

Introduce checkAuth so callers can verify Endorser acceptance of a JWT without dealing with HTTP details.

src/services/endorserClient.ts

@@ -0,0 +1,44 @@
+import { ENDORSER_URL } from "../env.js";
+import { errorMessage } from "../util/formatElapsed.js";
+
+const RATE_LIMITS_PATH = "/api/report/rateLimits";
+
+function rateLimitsUrl(): string {
+  const base = ENDORSER_URL.replace(/\/+$/, "");
+  return `${base}${RATE_LIMITS_PATH}`;
+}
+
+/**
+ * Confirms a JWT is accepted by the Endorser server.
+ *
+ * Calls GET /api/report/rateLimits with the JWT as a Bearer token.
+ * Returns true on success, false on auth or request failure.
+ * Does not expose HTTP status or response bodies to callers.
+ */
+export async function checkAuth(jwt: string): Promise<boolean> {
+  let response: Response;
+  try {
+    response = await fetch(rateLimitsUrl(), {
+      method: "GET",
+      headers: {
+        Authorization: `Bearer ${jwt}`,
+      },
+    });
+  } catch (err) {
+    console.error(
+      "[Endorser] Auth check request failed:",
+      errorMessage(err)
+    );
+    return false;
+  }
+
+  if (!response.ok) {
+    console.error(
+      "[Endorser] Auth check failed with status",
+      response.status
+    );
+    return false;
+  }
+
+  return true;
+}