feat(e2e): add payment fixtures for VNPay and MoMo callback testing

Add buildVnpayCallbackData and buildMomoCallbackData fixture helpers
that generate valid HMAC signatures for E2E payment callback tests.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

e2e/fixtures/index.ts

@@ -2,3 +2,4 @@ export { test, expect } from './auth.fixture';
 export { createTestUser, registerUser, loginUser } from './auth.fixture';
 export type { TokenPair } from './auth.fixture';
 export { createTestListing, createListing } from './listings.fixture';
+export { buildVnpayCallbackData, buildMomoCallbackData } from './payments.fixture';