feat(ladder): phase 4 - Azure (BillingBenefits savings plans + reservation exchange) #28

Part of LeanerCloud/cloud-commitments-cli#1333 (commitment laddering automation). Phase 4 of 4: Azure implementation (Azure has both needed instruments: exchangeable Reservations and Savings Plans for Compute).

PRs:

1. Azure read side: list existing savings plans via Microsoft.BillingBenefits; savings-plan utilization via Cost Management benefitUtilizationSummaries; usage baseline from Cost Management series. Reservation utilization already exists (getRIUtilizationViaAPI).
2. Azure write side: new savings-plan purchase client (providers/azure/services/savingsplan/, Microsoft.BillingBenefits/savingsPlanOrders, validate-then-put, api-version confirmed against current docs) and reservation exchange client (POST /providers/Microsoft.Capacity/calculateExchange -> POST /providers/Microsoft.Capacity/exchange, async operation polling) with the same spend-cap guardrails as the AWS exchange path. All raw-HTTP clients MUST use httpclient.New() (SSRF/IMDS blocking).
3. AzureLadder capability + enablement: implements LadderCapability with 2 layers (Reservation = base+buffer, SavingsPlan = flex); pipeline enablement for Azure scopes.