fix(platform): restore platform-vercel next config test under platforms/

The prior commit accidentally dropped this test during the directory
rename rather than moving it alongside the other platforms/vercel
files.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: ovflowd

platforms/vercel/__tests__/next.platform.config.test.mjs

@@ -0,0 +1,13 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+describe('platform-vercel next.platform.config', () => {
+  it('defines shiki mdx defaults for Vercel builds', async () => {
+    const { default: platform } = await import('../next.platform.config.mjs');
+
+    assert.deepEqual(platform.mdx, {
+      wasm: true,
+      twoslash: true,
+    });
+  });
+});