Function validate_response 

pub fn validate_response(
    resp: &SystemResponse,
    criteria: &HashMap<String, Vec<String>>,
) -> ValidationResult<()>

Validate a response. Catches the common engine-side bugs:

• missing or unexpected answers (when criteria covers the request’s question ids)
• probabilities don’t sum to 1
• confidence out of range
• choice answer’s keys don’t match criteria
• score answer legend doesn’t match probabilities

criteria maps question id → the question’s choice-criteria keys (empty for noul/score questions). When the map is non-empty it is treated as the full question-id set of the originating request, and answer coverage is enforced both ways: every requested id must be answered, and no extra answers may appear. An empty map skips coverage checks (per-answer checks still run).