Fix noise test teardown: stub AudioContext needs close()

The service's willDestroy() closes the context on teardown; the running
context stub lacked close(), so the test rejected with 'this.context.close
is not a function'. Mirrors the stub used in the playback-rate tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lifeart

frontend/tests/unit/services/audio-test.js

@@ -336,8 +336,13 @@ module('Unit | Service | audio', function (hooks) {
       this.owner.register('service:audio', TestAudioService);
       const service = this.owner.lookup('service:audio');
       // A running context is the post-restart case; the suspended-context
-      // resume is production-only (guarded by !isTesting()).
-      service.context = { state: 'running', resume: () => Promise.resolve() };
+      // resume is production-only (guarded by !isTesting()). close() is needed
+      // because the service's willDestroy() closes the context on teardown.
+      service.context = {
+        state: 'running',
+        resume: () => Promise.resolve(),
+        close: () => Promise.resolve(),
+      };
 
       const instance = service.startNoiseTask.perform();
       // The task loops on timeout(6000) after starting; cancel once started.