feat(FN-2322): centralize loopback integration test gating

- Add a shared createLoopbackIntegrationTest helper that probes 127.0.0.1 binding once and caches the result
- Use the helper in webhook, websocket, and static asset integration tests to replace duplicated loopback detection logic
- Improve skip diagnostics by including a consistent skip reason and integration scope in skipped test names

packages/dashboard/src/__tests__/loopback-integration-test.ts

@@ -0,0 +1,36 @@
+import http from "node:http";
+import { it } from "vitest";
+
+type IntegrationTestCase = (name: string, fn: () => unknown | Promise<unknown>, timeout?: number) => ReturnType<typeof it>;
+
+const LOOPBACK_SKIP_REASON = "loopback binding to 127.0.0.1 is unavailable in this environment";
+
+let loopbackBindingAvailablePromise: Promise<boolean> | null = null;
+
+async function detectLoopbackBinding(): Promise<boolean> {
+  return await new Promise((resolve) => {
+    const server = http.createServer();
+    server.once("error", () => resolve(false));
+    server.listen(0, "127.0.0.1", () => {
+      server.close(() => resolve(true));
+    });
+  });
+}
+
+async function isLoopbackBindingAvailable(): Promise<boolean> {
+  if (!loopbackBindingAvailablePromise) {
+    loopbackBindingAvailablePromise = detectLoopbackBinding();
+  }
+
+  return await loopbackBindingAvailablePromise;
+}
+
+export async function createLoopbackIntegrationTest(scope: string): Promise<IntegrationTestCase> {
+  const loopbackBindingAvailable = await isLoopbackBindingAvailable();
+
+  if (loopbackBindingAvailable) {
+    return (name, fn, timeout) => it(name, fn, timeout);
+  }
+
+  return (name, fn, timeout) => it.skip(`${name} (skipped: ${LOOPBACK_SKIP_REASON}; scope: ${scope})`, fn, timeout);
+}