bug: implement Flutterwave refunds and remove dead NotImplementedException path

[teesofttech]

Why

FlutterwaveGateway.RefundPaymentAsync throws NotImplementedException, even though refund support is part of IPaymentGateway and the README claims a full create/verify/refund lifecycle for all gateways. There is also an extra private unimplemented method.

Scope

• Implement refund creation using the supported Flutterwave API and correct transaction identifier.
• Map provider statuses and error responses into RefundResponse.
• Remove the unused unimplemented method.
• Define full versus partial refund behavior and amount units.
• Add mocked unit tests and sandbox integration tests.

Acceptance criteria

• RefundPaymentAsync no longer throws NotImplementedException.
• Successful, rejected, malformed, timeout, full, and partial refund cases are covered.
• The implementation uses IHttpClientFactory and does not log secrets.
• Sandbox evidence is captured in CI without exposing credentials.
• Capability documentation reflects actual support.