test: Unit & integration tests for PaystackGateway

[teesofttech]

Overview

Add a complete unit and integration test suite for PaystackGateway.

Gateway Details

• Class: PaystackGateway
• Auth: Bearer token (SecretKey)
• Base URL: https://api.paystack.co
• Currency: NGN (amount x100 kobo)
• Enum: PaymentGatewayType.Paystack = 2

Unit Tests (mocked HTTP)

Constructor

• Throws InvalidOperationException when SecretKey is null/empty
• Constructs successfully with a valid SecretKey

CreatePaymentAsync

• Posts to /transaction/initialize with amount in kobo
• Sets Authorization: Bearer header
• Returns PaymentResponse.Success=true and CheckoutUrl on 200 success response
• Returns PaymentResponse.Success=false on API error
• Handles HttpRequestException gracefully

VerifyPaymentAsync

• Sends GET /transaction/verify/{reference}
• Returns IsSuccessful=true when data.status=success
• Returns IsSuccessful=false when data.status=failed

RefundPaymentAsync

• Posts to /refund with correct body
• Returns RefundResponse.IsSuccessful=true on success

Integration Tests

Requires PAYSTACK_SECRET_KEY env var (sandbox key)

• CreatePaymentAsync returns a real Paystack checkout URL
• VerifyPaymentAsync returns correct status for a known sandbox reference

Files to Create

• PayBridge.SDK.Test/Unit/PaystackGatewayTests.cs
• PayBridge.SDK.Test/Integration/PaystackIntegrationTests.cs

Acceptance Criteria

• All unit tests pass with zero live HTTP calls
• Integration tests decorated with [Trait("Category", "Integration")] and skip when env var is absent
• Code coverage >= 80% for PaystackGateway.cs