runtime: normalize structured secret errors

crates/crank-runtime/src/auth.rs

@@ -92,7 +92,7 @@ fn extract_secret_string(
     let Some(object) = value.as_object() else {
         return Err(RuntimeError::InvalidAuthSecretValue {
             secret_id: secret_id.as_str().to_owned(),
-            details: "secret payload must be a string or object".to_owned(),
+            reason: "secret payload must be a string or object".to_owned(),
         });
     };
 
@@ -104,7 +104,7 @@ fn extract_secret_string(
 
     Err(RuntimeError::InvalidAuthSecretValue {
         secret_id: secret_id.as_str().to_owned(),
-        details: format!(
+        reason: format!(
             "secret payload must contain one of: {}",
             preferred_fields.join(", ")
         ),