[kotlin-server] add support for delegate pattern

Is your feature request related to a problem? Please describe.

The current Kotlin Javalin generator creates a handler that directly calls a service, expecting the service to either return successfully or throw an exception. This approach prevents the handler from performing custom input validation or pre-processing, or handling multiple 2xx status code e.g 200 or 204 status meaning all errors must be managed through exception handling in the service.

Describe the solution you'd like

A more flexible approach would allow the handler to validate and format request data before delegating to the service, without relying solely on exceptions for error handling. This could involve a handler interface with methods that enable custom validation and pre-processing.

Describe alternatives you've considered

Another option is to create custom controllers and bypass the generated ones. However, this approach would prevent the OpenAPI definition from automatically syncing with the custom code.

Start by locating the Kotlin Javalin server generator and the generated handler/service integration described in the issue. Review how request validation, preprocessing, service delegation, and multiple 2xx responses are currently represented; done means a documented delegate pattern that preserves synchronization with the OpenAPI definition.