test(opensearch): pin the wire encoding of reserved-instance purchases after the SDK serializer change #89

Follow-up from the review of #88, the Dependabot gomod bump.

aws-sdk-go-v2/service/opensearch went from v1.52.3 to v1.85.1, and smithy-go from v1.24.2 to v1.28.5. The OpenSearch client now serializes requests through the new schema-based engine and the restjson1 protocol. The PurchaseReservedInstanceOffering input struct is unchanged, with the same fields and types, and the other five purchase services keep their old serializers.

This repo's tests mock the provider client interface, so nothing checks the wire encoding of InstanceCount, ReservationName and ReservedInstanceOfferingId on the new path.

Suggested fix: add an httptest-backed test that points the OpenSearch client at a local server. It should capture the serialized PurchaseReservedInstanceOffering request body and assert the JSON field names and values, including InstanceCount. No real AWS calls. This may belong in cloud-commitments-go's opensearch provider, which owns the client.