Fix tests broken by Vitest v4 and React 19 upgrades

Vitest v4 changed how vi.fn() handles `new` calls: when a mock has an
arrow-function implementation, calling it with `new` now routes through
Reflect.construct which rejects arrow functions as constructors. Replace
all arrow-function implementations in Image mocks with regular functions.

Also fix the duplicate-script-prevention test: in Vitest v4, vi.spyOn on
an already-spied method returns the same spy object (shared call history),
so the previous appendSpy approach incorrectly counted pre-existing calls.
Replaced with mockClear() + re-check on the original spy.

Author: JArmandoAnaya

tests/OpenCVCanvas.test.tsx

@@ -62,7 +62,7 @@ describe('OpenCVCanvas', () => {
     it('calls onError when image fails to load', async () => {
       // Override Image mock to trigger error instead of load
       const OriginalImage = global.Image;
-      global.Image = vi.fn().mockImplementation(() => {
+      global.Image = vi.fn(function(this: unknown) {
         const img: Partial<HTMLImageElement> = {};
         Object.defineProperty(img, 'src', {
           set() {
@@ -209,7 +209,7 @@ describe('OpenCVCanvas', () => {
       const OriginalImage = global.Image;
       const triggers: Array<() => void> = [];
 
-      global.Image = vi.fn().mockImplementation(() => {
+      global.Image = vi.fn(function(this: unknown) {
         const img: Partial<HTMLImageElement> = {
           naturalWidth: 100,
           naturalHeight: 100,

tests/setup.ts

@@ -42,7 +42,10 @@ HTMLCanvasElement.prototype.getContext = vi.fn(
 export function mockImageLoad(naturalWidth = 100, naturalHeight = 100) {
   const originalImage = global.Image;
 
-  const MockImage = vi.fn().mockImplementation(() => {
+  // Regular function (not arrow) required: vi.fn() called with `new` in Vitest v4
+  // passes the constructor call through to the implementation, and arrow functions
+  // cannot be constructors.
+  const MockImage = vi.fn(function MockImageImpl(this: unknown) {
     const img: Partial<HTMLImageElement> & { _triggerLoad?: () => void; _triggerError?: () => void } = {
       naturalWidth,
       naturalHeight,

tests/useOpenCV.test.tsx

@@ -105,21 +105,22 @@ describe('useOpenCV', () => {
         await r1.current.load();
       });
 
-      // Second hook instance should not inject another script
-      const appendSpy = vi.spyOn(document.head, 'appendChild');
+      // Second hook instance should not inject another script.
+      // In Vitest v4, vi.spyOn on an already-spied method returns the same spy
+      // object (shared call history), so we verify via the DOM instead.
+      spy.mockClear();
       const { result: r2 } = renderHook(() => useOpenCV({ autoLoad: false }));
       await act(async () => {
         await r2.current.load();
       });
 
-      // No new script appended
-      const scriptAppends = appendSpy.mock.calls.filter(
+      // No new script appended after the clear point
+      const scriptAppends = spy.mock.calls.filter(
         ([node]) => node instanceof HTMLScriptElement,
       );
       expect(scriptAppends).toHaveLength(0);
 
       spy.mockRestore();
-      appendSpy.mockRestore();
     });
   });