Some request fields are not omitted

[gordon2]

Description of the Bug

When you run request /estimates/update_subscription_for_items with remove discount param you end up with an error.

This happens because two of the request fields are missing 'omitempty' param in the struct UpdateSubscriptionForItemsDiscountParams

	ApplyOn       enum.ApplyOn       `json:"apply_on"`
	DurationType  enum.DurationType  `json:"duration_type"`

Steps to reproduce

run this code

chargebee.Configure("", "")

subID := "subscription-id"
discountID := "id-of-existing-discount"

_, err := estimate.UpdateSubscriptionForItems(&estimateModels.UpdateSubscriptionForItemsRequestParams{
	Subscription: &estimateModels.UpdateSubscriptionForItemsSubscriptionParams{
		Id: subID,
	},
	Discounts: []*estimateModels.UpdateSubscriptionForItemsDiscountParams{
		{
			Id:            discountID,
			OperationType: enum.OperationTypeRemove,
		},
	},
}).Request()

if err != nil {
	fmt.Println(err)
}

get the following response
{"http_status_code":400,"message":"discounts[apply_on][0] : should not be sent when operation type is REMOVE","param":"discounts[apply_on][1]","api_error_code":"invalid_request","type":"invalid_request","error_code":"param_should_not_be_sent","_":{}}

Expected Behavior

response should return 200 with estimates data

Code Snippets (if applicable)

Operating System

macOS

Language version

Go 1.24

Library version

v3.29.0

Additional context

No response