Jackson in Spring Boot: Serialize a flat Map (adjacency list) into a nested JSON tree without intermediate objects [closed]

I am building a Spring Boot REST API where a strict architectural constraint requires me to implement and expose a tree data structure using only standard Java Collections (e.g., Map, List). I am explicitly forbidden from creating custom Node, Tree, or intermediate DTO classes for this specific engine implementation. The OpenAPI contract expects a deeply nested JSON response, but my tree is maintained in memory as flat maps.