feat(wangapayfast-rs): introduce typed responses for API client endpoints

[mitch1009]

Summary

Replace raw serde_json::Value API returns with typed response structs for safer downstream usage.

Scope

• Define typed structs for subscription, transactions, refunds, and process query endpoints
• Keep optional raw JSON access for forward compatibility
• Update client methods to return typed responses

Acceptance Criteria

• Public API methods return typed models
• Serialization/deserialization tests added
• Backward compatibility strategy documented