I read that schema validation is available in Kafka Confluent Server only. But maybe since then someone has found a solution? Does anyone know how can I test message validity running Kafka locally?
how to enforce broker to validate producers' input
Kafka doesn't do this. Confluent Server can, yes, but it is enterprise licensed. There is no alternative to server-side validation without forking Kafka like Confluent did. Otherwise, you would need to write your own Serializer class, but that won't scale to every producer client you may use.
You can still use Docker as the other answer shows, and you can use confluentinc/cp-server image, then you simply add an environment variable to enable schema valdiation.
https://docs.confluent.io/platform/current/schema-registry/schema-validation.html
Otherwise, simply using JsonSchemaSerializer, for example, will validate each record does adhere to a schema (client-side validation), but it won't stop anyone else from sending garbage data into the topic (server-side validation).