refactor: Update handler constructors to use interface types instead of concrete implementations

- Update CurrencyHandler.exchangeService field from *currency.ExchangeRateService to currency.ExchangeRateService interface
- Update NewCurrencyHandler parameter from *currency.ExchangeRateService to currency.ExchangeRateService interface
- Update RatePlanHandler.service field from *rateplan.Service to rateplan.Service interface
- Update NewRatePlanHandler parameter from *rateplan.Service to rateplan.Service interface

apps/carrier-connector/internal/handlers/currency_handlers.go

@@ -14,12 +14,12 @@ import (
 // CurrencyHandler handles currency-related HTTP requests
 type CurrencyHandler struct {
 	billingService  currency.BillingService
-	exchangeService *currency.ExchangeRateService
+	exchangeService currency.ExchangeRateService
 	logger          *logrus.Logger
 }
 
 // NewCurrencyHandler creates a new currency handler
-func NewCurrencyHandler(billingService currency.BillingService, exchangeService *currency.ExchangeRateService, logger *logrus.Logger) *CurrencyHandler {
+func NewCurrencyHandler(billingService currency.BillingService, exchangeService currency.ExchangeRateService, logger *logrus.Logger) *CurrencyHandler {
 	return &CurrencyHandler{
 		billingService:  billingService,
 		exchangeService: exchangeService,

apps/carrier-connector/internal/handlers/rateplan_handlers_core.go

@@ -11,12 +11,12 @@ import (
 
 // RatePlanHandler handles rate plan API endpoints
 type RatePlanHandler struct {
-	service *rateplan.Service
+	service rateplan.Service
 	logger  *logrus.Logger
 }
 
 // NewRatePlanHandler creates a new rate plan handler
-func NewRatePlanHandler(service *rateplan.Service) *RatePlanHandler {
+func NewRatePlanHandler(service rateplan.Service) *RatePlanHandler {
 	logger := logrus.New()
 	logger.SetLevel(logrus.InfoLevel)