feat: define ML service API responses

apps/ml-service/src/error.rs

@@ -0,0 +1,77 @@
+use axum::{
+    http::StatusCode,
+    response::{IntoResponse, Response},
+    Json,
+};
+use serde_json::json;
+
+#[derive(Debug)]
+pub enum ServiceError {
+    BadRequest(String),
+    ModelUnavailable(String),
+    PredictionFailed(String),
+}
+
+impl IntoResponse for ServiceError {
+    fn into_response(self) -> Response {
+        let (status, detail) = match self {
+            ServiceError::BadRequest(msg) => (StatusCode::BAD_REQUEST, msg),
+            ServiceError::ModelUnavailable(msg) => (StatusCode::SERVICE_UNAVAILABLE, msg),
+            ServiceError::PredictionFailed(msg) => (StatusCode::INTERNAL_SERVER_ERROR, msg),
+        };
+
+        let body = Json(json!({ "detail": detail }));
+        (status, body).into_response()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use axum::body::to_bytes;
+
+    #[tokio::test]
+    async fn bad_request_maps_to_400() {
+        let error = ServiceError::BadRequest("invalid input".to_string());
+        let response = error.into_response();
+
+        assert_eq!(response.status(), StatusCode::BAD_REQUEST);
+
+        let body = to_bytes(response.into_body(), usize::MAX)
+            .await
+            .expect("failed to read body");
+        let json: serde_json::Value = serde_json::from_slice(&body).expect("invalid json");
+
+        assert_eq!(json["detail"], "invalid input");
+    }
+
+    #[tokio::test]
+    async fn model_unavailable_maps_to_503() {
+        let error = ServiceError::ModelUnavailable("model not loaded".to_string());
+        let response = error.into_response();
+
+        assert_eq!(response.status(), StatusCode::SERVICE_UNAVAILABLE);
+
+        let body = to_bytes(response.into_body(), usize::MAX)
+            .await
+            .expect("failed to read body");
+        let json: serde_json::Value = serde_json::from_slice(&body).expect("invalid json");
+
+        assert_eq!(json["detail"], "model not loaded");
+    }
+
+    #[tokio::test]
+    async fn prediction_failed_maps_to_500() {
+        let error = ServiceError::PredictionFailed("inference error".to_string());
+        let response = error.into_response();
+
+        assert_eq!(response.status(), StatusCode::INTERNAL_SERVER_ERROR);
+
+        let body = to_bytes(response.into_body(), usize::MAX)
+            .await
+            .expect("failed to read body");
+        let json: serde_json::Value = serde_json::from_slice(&body).expect("invalid json");
+
+        assert_eq!(json["detail"], "inference error");
+    }
+}