fix: use act instead of waitFor in fake-timer test for agent fade-out

waitFor's internal polling setInterval is itself faked when vi.useFakeTimers()
is active, causing it to hang indefinitely. Replace the affected assertions
with act + multiple Promise.resolve() to flush React state updates, and a
direct assertion after act+vi.advanceTimersByTime instead of a second waitFor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AgentPanel.test.tsx

@@ -1,4 +1,4 @@
-import { render, screen, waitFor } from "@testing-library/react";
+import { act, render, screen, waitFor } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import {
 	afterEach,
@@ -345,20 +345,27 @@ describe("AgentPanel fade-out", () => {
 
 			const { container } = render(<AgentPanel />);
 
-			// Wait for the agent entry to appear
+			// With fake timers active, waitFor's polling setInterval never fires.
-			await waitFor(() => {
+			// Use act to flush pending promises and React state updates instead.
+			await act(async () => {
+				await Promise.resolve();
+				await Promise.resolve();
+				await Promise.resolve();
+				await Promise.resolve();
+			});
+
 			expect(
 				container.querySelector(
 					'[data-testid="agent-entry-73_remove_test:coder-1"]',
 				),
 			).toBeInTheDocument();
+
+			// Advance fake timers by 60 seconds inside act to flush React state updates
+			await act(async () => {
+				vi.advanceTimersByTime(60_000);
 			});
 
-			// Advance timers by 60 seconds
-			vi.advanceTimersByTime(60_000);
-
 			// Entry should be removed
-			await waitFor(() => {
 			expect(
 				container.querySelector(
 					'[data-testid="agent-entry-73_remove_test:coder-1"]',
@@ -366,5 +373,4 @@ describe("AgentPanel fade-out", () => {
 			).not.toBeInTheDocument();
 		});
 	});
-	});
 });