Azure: register Microsoft.BillingBenefits resource provider in custom role

[cristim]

Summary

The custom role added by PR #744 (and now shared via terraform/modules/iam/azure/cudly-reservation-role) grants Microsoft.Capacity/register/action but not Microsoft.BillingBenefits/register/action. On a customer subscription that has never used Azure Savings Plans, the resource provider Microsoft.BillingBenefits may not be registered. The first Savings Plan order in that subscription will return a 409 MissingSubscriptionRegistration error until the provider is registered out-of-band.

Repro

1. Use a "Savings Plan virgin" customer subscription (never created a Savings Plan before).
2. Attempt to create a Savings Plan via CUDly's two-step calculatePrice -> purchase flow.
3. Observe 409 MissingSubscriptionRegistration for Microsoft.BillingBenefits.

Fix

Append Microsoft.BillingBenefits/register/action to the actions list in both:

• terraform/modules/iam/azure/cudly-reservation-role/main.tf (shared Terraform module)
• arm/CUDly-CrossSubscription/template.json (customer ARM template)

Workaround

Until fixed, the customer administrator can run the following once before onboarding:

az provider register --namespace Microsoft.BillingBenefits

Registration is idempotent and typically completes within a few minutes.

[cristim]

Verified shipped: PR #759 (commit 3249775, merged 2026-05-27) is in feat/multicloud-web-frontend. Microsoft.BillingBenefits/register/action added to both arm/CUDly-CrossSubscription/template.json and terraform/modules/iam/azure/cudly-reservation-role/main.tf.