frontend: multiplexer: cancel pending timers in test teardown

The afterEach in multiplexer.test.ts called
pendingUnsubscribes.clear() without cancelling the stored timeouts.
This left orphaned setTimeout callbacks that could fire after vitest
tore down the jsdom environment, causing a sporadic
"ReferenceError: WebSocket is not defined".

Call forEach(clearTimeout) before clear() so all pending debounce
timers are cancelled before the test environment is destroyed.

Author: Kaniikura

frontend/src/lib/k8s/api/v2/multiplexer.test.ts

@@ -89,6 +89,7 @@ describe('WebSocket Multiplexer', () => {
     WebSocketManager.listeners.clear();
     WebSocketManager.completedPaths.clear();
     WebSocketManager.activeSubscriptions.clear();
+    WebSocketManager.pendingUnsubscribes.forEach(clearTimeout);
     WebSocketManager.pendingUnsubscribes.clear();
   });