get rid of mockImplementation(() => promise) - causing memory leaks

src/features/charts/PriceHistoryChart.test.tsx

@@ -1,6 +1,6 @@
 // src/components/PriceHistoryChart.test.tsx
 import React from 'react';
-import { render, screen, waitFor } from '@testing-library/react';
+import { render, screen, waitFor, act } from '@testing-library/react';
 import { describe, it, expect, vi, beforeEach, type Mock } from 'vitest';
 import { PriceHistoryChart } from './PriceHistoryChart';
 import * as apiClient from '../../services/apiClient';
@@ -56,11 +56,19 @@ describe('PriceHistoryChart', () => {
     vi.clearAllMocks();
   });
 
-  it('should render a loading spinner while fetching data', () => {
-    (apiClient.fetchHistoricalPriceData as Mock).mockReturnValue(new Promise(() => {}));
+  it('should render a loading spinner while fetching data', async () => {
+    let resolvePromise: (value: Response) => void;
+    const mockPromise = new Promise<Response>(resolve => {
+      resolvePromise = resolve;
+    });
+    (apiClient.fetchHistoricalPriceData as Mock).mockReturnValue(mockPromise);
     render(<PriceHistoryChart watchedItems={mockWatchedItems} />);
     expect(screen.getByText(/loading price history/i)).toBeInTheDocument();
     expect(screen.getByRole('status')).toBeInTheDocument(); // LoadingSpinner
+    await act(async () => {
+      resolvePromise(new Response(JSON.stringify([])));
+      await mockPromise;
+    });
   });
 
   it('should render an error message if fetching fails', async () => {