chore(site): clean up mocks after each test (#12805)

site/jest.polyfills.js

@@ -29,4 +29,16 @@ Object.defineProperties(globalThis, {
   FormData: { value: FormData },
   Request: { value: Request },
   Response: { value: Response },
+  matchMedia: {
+    value: (query) => ({
+      matches: false,
+      media: query,
+      onchange: null,
+      addListener: jest.fn(),
+      removeListener: jest.fn(),
+      addEventListener: jest.fn(),
+      removeEventListener: jest.fn(),
+      dispatchEvent: jest.fn(),
+    }),
+  },
 });