runtime: normalize structured secret errors

crates/crank-runtime/src/executor.rs

@@ -395,7 +395,8 @@ impl PreparedRequest {
             mapped
                 .get("request")
                 .ok_or_else(|| RuntimeError::InvalidPreparedRequest {
-                    details: "mapped input must contain request root".to_owned(),
+                    field: "request".to_owned(),
+                    reason: "mapped input must contain request root".to_owned(),
                 })?;
 
         Ok(Self {
@@ -460,7 +461,8 @@ fn read_string_map(
 
     let Some(object) = value.as_object() else {
         return Err(RuntimeError::InvalidPreparedRequest {
-            details: format!("{field_name} must be an object"),
+            field: field_name.to_owned(),
+            reason: "must be an object".to_owned(),
         });
     };
 
@@ -468,7 +470,10 @@ fn read_string_map(
         .iter()
         .map(|(key, value)| stringify_value(value).map(|value| (key.clone(), value)))
         .collect::<Result<BTreeMap<_, _>, _>>()
-        .map_err(|details| RuntimeError::InvalidPreparedRequest { details })
+        .map_err(|reason| RuntimeError::InvalidPreparedRequest {
+            field: field_name.to_owned(),
+            reason,
+        })
 }
 
 fn stringify_value(value: &Value) -> Result<String, String> {
@@ -525,7 +530,7 @@ mod tests {
     use tokio::net::TcpListener;
     use tokio_tungstenite::accept_async;
 
-    use crate::{RuntimeError, RuntimeExecutor, RuntimeOperation};
+    use crate::{PreparedRequest, RuntimeError, RuntimeExecutor, RuntimeOperation};
 
     fn timestamp(value: &str) -> OffsetDateTime {
         OffsetDateTime::parse(value, &Rfc3339).unwrap()
@@ -638,6 +643,19 @@ mod tests {
         assert!(matches!(error, RuntimeError::Schema(_)));
     }
 
+    #[test]
+    fn rejects_missing_request_root_with_structured_error() {
+        let error = PreparedRequest::from_mapping_output(&json!({})).unwrap_err();
+
+        match error {
+            RuntimeError::InvalidPreparedRequest { field, reason } => {
+                assert_eq!(field, "request");
+                assert_eq!(reason, "mapped input must contain request root");
+            }
+            other => panic!("unexpected error: {other}"),
+        }
+    }
+
     #[tokio::test]
     async fn propagates_external_rest_errors() {
         let base_url = spawn_runtime_server().await;