test: reset gateway in afterAll in cli-multimodal-integration — cross-file config leak

Review hardening on #3116: the file resets the gateway in beforeEach but
left it configured after the last test, leaking a multimodal gateway
config into whatever file runs next in its shard (same class as the
adaptive-embed-batch leak fixed on master).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/cli-multimodal-integration.test.ts

@@ -28,6 +28,7 @@ beforeAll(async () => {
 });
 
 afterAll(async () => {
+  resetGateway(); // don't leak this file's gateway config into shard siblings
   await engine.disconnect();
 });