How to model Lambda cold-start behaviour under spike traffic before you deploy

There is a class of AWS incident I have started calling the "everything looked fine in testing" failure. The pattern is consistent. You design a serverless API. Lambda function with sensible defaults, wired through API Gateway, pointing at DynamoDB. You test it in dev throughout the week. Latency is acceptable. Costs track to plan. Then a campaign drops, or a new enterprise customer brings their three thousand users on day one, and your traffic goes from 300 RPS to 3,000 RPS in under a minute. L