feat(multiple-payment-methods): add invoice custom sections to billing resources

[aquinofb]

Summary

The Lago backend now supports attaching invoice custom sections to additional billing resources beyond customer and billing entity. This PR updates the Go API client to expose those new fields on subscription, wallet, recurring transaction rule, wallet transaction, and one-off invoice.

On the request side, each resource now accepts an optional nested invoice_custom_section object containing invoice_custom_section_codes (an array of section code strings) and skip_invoice_custom_sections (a boolean flag). This nesting mirrors the backend's strong params structure, which is notably different from the customer resource, where these fields sit at the top level.

On the response side, each resource now deserializes an applied_invoice_custom_sections array. Each entry contains lago_id, invoice_custom_section_id, and created_at: the join-table representation returned by the backend's AppliedInvoiceCustomSectionSerializer, not the full InvoiceCustomSection object.

Two new shared types were introduced in invoice_custom_section.go: InvoiceCustomSectionInput for requests and AppliedInvoiceCustomSection for responses. This keeps the pattern consistent across all five resources without duplicating struct definitions.

The implementation was cross-verified against the backend controller strong params, serializer output, and the equivalent Python client PR (getlago/lago-python-client#382) to ensure the JSON field names and nesting structure match exactly.

How to test it locally

Pull the Go Docker image if you don't have Go installed:

docker pull golang:latest

Run the tests:

docker run --rm -v $(pwd):/app -w /app golang:latest go test ./...

To verify the request JSON shape against a local Lago instance, you can spin up an echo server and point the client at it:

# Terminal 1: start an echo server
docker run --rm -p 8080:80 mendhak/http-https-echo

# Terminal 2: run a small Go script that creates a subscription with custom sections
docker run --rm --network host -v $(pwd):/app -w /app golang:latest go run -<<'GOEOF'
package main

import (
    "context"
    "fmt"
    lago "github.com/getlago/lago-go-client"
)

func main() {
    client := lago.New().SetBaseURL("http://localhost:8080").SetApiKey("test")
    client.Subscription().Create(context.Background(), &lago.SubscriptionInput{
        ExternalCustomerID: "cust_1",
        PlanCode:           "premium",
        ExternalID:         "sub_1",
        Name:               "Test",
        BillingTime:        lago.Anniversary,
        InvoiceCustomSection: &lago.InvoiceCustomSectionInput{
            InvoiceCustomSectionCodes: []string{"section_1", "section_2"},
            SkipInvoiceCustomSections: true,
        },
    })
    fmt.Println("Check the echo server output for the request JSON")
}
GOEOF

The echo server will print the full request body. Verify that invoice_custom_section_codes and skip_invoice_custom_sections are nested under an invoice_custom_section key.

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[osmarluz]

@aquinofb not a blocker, but it would be good to update the customer_wallet_test.go to add the tests we have here, since they are shared by the customer_wallet.go endpoints too

[aquinofb]

Good call, Osmar. Brought customer_wallet_test.go to full parity with wallet_test.go in f9f9927. The mock fixtures now include payment_method and applied_invoice_custom_sections, the Get test asserts on those fields, and the new CreateWithPaymentMethod and UpdateWithPaymentMethod test cases cover the same scenarios as the wallet endpoint.

[osmarluz]

🚀