feat: finish donor-led custom endpoint integration

tests/helpers/custom-provider-auth.ts

@@ -0,0 +1,10 @@
+/** Synthetic authorized records, imported only after each test's isolated env. */
+import { randomBytes } from "node:crypto";
+import type { CustomProviderSettings } from "../../src/core/settings.ts";
+import { persistCustomAuth } from "../../src/server/custom-provider-auth.ts";
+
+export function authorizeFixture(cp: CustomProviderSettings, fileKey?: string): CustomProviderSettings {
+	const id = randomBytes(16).toString("hex");
+	persistCustomAuth({ version: 1, id, baseUrl: cp.baseUrl, authMode: cp.authMode ?? "keyless", ...(fileKey ? { fileKeyBaseUrl: cp.baseUrl } : {}) }, fileKey);
+	return { ...cp, credentialBindingId: id };
+}