fix: restoring mocked function properly

Author: aryanjassal

tests/auth/login.test.ts

@@ -27,7 +27,7 @@ describe('commandAuthLogin', () => {
   let polykeyAgent: PolykeyAgent;
 
   beforeEach(async () => {
-    globalThis.fetch = jest.fn(() =>
+    jest.spyOn(globalThis, 'fetch').mockImplementation(() =>
       Promise.resolve(
         new Response(JSON.stringify({ result: 'success' }), {
           status: 200,