fix: update json mock to be async in authentication tests

Author: hanna-skryl

__tests__/auth.test.ts

@@ -16,7 +16,7 @@ describe('authenticate', () => {
     mockFetch.mockResolvedValue({
       ok: true,
       status: 200,
-      json: () => Promise.resolve({ token: 'ghs_app_123' })
+      json: async () => Promise.resolve({ token: 'ghs_app_123' })
     } as Response)
 
     const result = await authenticate(
@@ -38,7 +38,7 @@ describe('authenticate', () => {
     mockFetch.mockResolvedValue({
       ok: false,
       status: 404,
-      json: () => Promise.resolve({ error: 'Not installed' })
+      json: async () => Promise.resolve({ error: 'Not installed' })
     } as Response)
 
     const result = await authenticate(