fix(custom-widgets): address v2 merge review (#6446)

scripts/check-custom-widget-bundle.spec.ts

@@ -0,0 +1,21 @@
+// @vitest-environment node
+
+import { spawnSync } from "node:child_process";
+import { describe, expect, test } from "vitest";
+
+describe("Custom Widget bundle budget", () => {
+  test.each(["not-a-number", "0", "-1", "1.5", String(Number.MAX_SAFE_INTEGER + 1)])(
+    "rejects an invalid byte budget (%s)",
+    (budget) => {
+      const result = spawnSync(process.execPath, ["scripts/check-custom-widget-bundle.mjs"], {
+        cwd: process.cwd(),
+        encoding: "utf8",
+        env: { ...process.env, CUSTOM_WIDGET_CHUNK_BUDGET_BYTES: budget },
+      });
+
+      expect(result.status).not.toBe(0);
+      expect(result.stderr).toContain("CUSTOM_WIDGET_CHUNK_BUDGET_BYTES must be a positive integer");
+      expect(result.stderr).not.toContain("Next.js build output is missing");
+    },
+  );
+});