feat(commitmentopts/azure): probe Azure Savings Plans offerings for live term/payment validation #659

PR #592 added the Azure Savings Plans provider.ServiceClient but deferred the commitmentopts Savings Plans probe equivalent for Azure. Today only AWS has a live Savings Plans offering probe (issue #134); Azure SP term/payment validation has no equivalent live-probe path.

PR #592 listed this under "Out of scope (follow-up issues filed below)" but no issue was actually filed. Capturing it now.

Scope:

• Add an Azure equivalent of the AWS commitmentopts Savings Plans probe so Azure SP term/payment options can be validated live (parallel to feat(commitmentopts): probe AWS Savings Plans offerings for live term/payment validation #134 for AWS).

Out of scope when deferred: PR #592 covered the provider client + dispatch wiring only.

Deferred by: #592
Related: #134 (AWS SP probe), #554 (Azure SP support), #473 (parity).