fix: Convert ProductIOS to dictionaries for Kotlin interop

- Cast Product protocol to concrete ProductIOS type before encoding
- Use JSONEncoder to serialize ProductIOS/ProductSubscriptionIOS
- Convert JSON to dictionaries for Kotlin compatibility
- Fixes nested structure issue in cross-language data transfer

Author: hyochan

Sources/OpenIapModule+ObjC.swift

@@ -43,11 +43,26 @@ import StoreKit
                 let request = ProductRequest(skus: skus, type: productType)
                 let result = try await fetchProducts(request)
 
+                let encoder = JSONEncoder()
+                encoder.dateEncodingStrategy = .millisecondsSince1970
+
                 switch result {
                 case .products(let products):
-                    completion(products, nil)
+                    // Cast to concrete type ProductIOS and convert to dictionaries
+                    let productIOS = products?.compactMap { $0 as? ProductIOS }
+                    let dictionaries = try productIOS?.compactMap { product -> [String: Any]? in
+                        let jsonData = try encoder.encode(product)
+                        return try JSONSerialization.jsonObject(with: jsonData) as? [String: Any]
+                    }
+                    completion(dictionaries, nil)
                 case .subscriptions(let subscriptions):
-                    completion(subscriptions, nil)
+                    // Cast to concrete type ProductSubscriptionIOS and convert to dictionaries
+                    let subscriptionIOS = subscriptions?.compactMap { $0 as? ProductSubscriptionIOS }
+                    let dictionaries = try subscriptionIOS?.compactMap { subscription -> [String: Any]? in
+                        let jsonData = try encoder.encode(subscription)
+                        return try JSONSerialization.jsonObject(with: jsonData) as? [String: Any]
+                    }
+                    completion(dictionaries, nil)
                 }
             } catch {
                 completion(nil, error)