chore(ri-exchange): forward the source RI availability zone to the go offering lookups #921

Summary

LeanerCloud/cloud-commitments-go#310 makes the AWS RI exchange offering lookups zone-aware:

• fix(ec2): filter RI exchange offering lookups by availability zone cloud-commitments-go#427 (provider): FindConvertibleOfferingParams and ListTargetOfferingsParams gain AvailabilityZone. A zonal scope without an AZ, or a regional scope with one, is refused before any API call. Only offerings in that zone are returned.
• feat(exchange)!: pass the source RI's availability zone to LookupOffering cloud-commitments-go#428 (pkg, breaking): exchange.LookupOfferingFunc takes a LookupOfferingParams struct, and RIMetadataInfo gains AvailabilityZone.

Needed here after bumping the go pins

1. Set RIMetadataInfo.AvailabilityZone from the source convertible RI (ConvertibleRI.AvailabilityZone) wherever RIMetadata is built for RunAutoExchange.
2. Adapt the LookupOffering closure to LookupOfferingParams and pass AvailabilityZone into FindConvertibleOfferingParams.
3. Pass the source AZ into ListTargetOfferingsParams on the manual exchange path.
4. Map the new zone refusals to a 4xx.

Until then, a zonal RI exchange fails loud with "zonal RI offering needs an availability zone". It never silently crosses zones.

Test: a zonal source RI in us-east-1b reaches the provider lookup with AvailabilityZone: "us-east-1b", and a regional one with none.