feat(aws/ladder): Convertible RI commitment rate for purchase sizing #732

Part of the ladder phase 3 work in cloud-commitments-platform#70 (the Convertible RI layer).

GetCommitmentRates returns a purchase-sizing rate per layer. Compute SP (#698) and EC2 Instance SP have one; the Convertible RI layer has none, so the engine holds the buffer share that this layer would receive (DefaultBufferFraction).

Add a Convertible RI commitment rate for purchase sizing:

• New recommendations.GetRICommitmentRate over GetReservationPurchaseRecommendation (EC2, Convertible, the same input as recommendations/client.go, using the ec2RecommendationOfferingClass constant from fix(aws): EC2 RI recommendation uses Standard pricing but the purchase defaults to Convertible #375), mirroring the Savings Plan formula in sp_commitment_rate.go: per detail, covered on-demand = effective monthly RI cost (recurring monthly cost plus upfront cost amortized over the term) plus estimated monthly savings, rate = utilization times cost divided by covered on-demand, as a ratio of sums; filter by account and region; strict parse; a sentinel error when no rate is present.
• Support all three payment options (converting existing coverage needs the global-term rate).
• Add LayerConvertibleRI to GetCommitmentRates, wired from the recommendations client beside the existing rate sources. A missing RI rate omits only that layer; any other error stays loud.
• Do not touch parser_ri.go (the fix(aws): RI recommendation count and costs disagree when Cost Explorer's quantity is fractional #635 lane).
• Fixture-based tests, including independence from the other two layers.

This is the sizing rate (marginal new purchase), not the observed rate of #686, which converts existing coverage.