fix(azure/savingsplans): purchase path sends PlanType "Compute" as sku.name; the API expects Compute_Savings_Plan #387

savingsplans.Client.PurchaseCommitment and buildValidateBody set SKU.Name to SavingsPlanDetails.PlanType. GetValidResourceTypes advertises "Compute"/"MachineLearning", and the new recommendations (#371, PR #373) also use "Compute". The Benefit Recommendations API names the SKU Compute_Savings_Plan (armSkuName), which is what the SavingsPlanOrderAlias sku.name expects.

Needs a decision on the PlanType vocabulary (map at purchase time vs. carry the ARM SKU name in the details). Not changed in #373. Verify against the SavingsPlanOrderAlias docs before changing; no purchase call should be made to test it.