test: provide subscription in billing context mock

Amp-Thread-ID: https://ampcode.com/threads/T-01a02479-3fb4-70b8-886b-39205e4a7c4d
Co-authored-by: Amp <amp@ampcode.com>

Author: dante01yoon

src/services/dialogService.topUpCredits.test.ts

@@ -47,6 +47,7 @@ vi.mock('@/composables/billing/useBillingContext', () => ({
     },
     isTeamPlan: { value: state.isTeamPlan },
     tier: { value: state.tier },
+    subscription: { value: null },
     type: { value: state.type }
   })
 }))