fix: preserve response body when error_status_codes triggers (#668)

Previously, when AWS_LWA_ERROR_STATUS_CODES matched a response status
code, the adapter discarded the HTTP response body and returned a
generic error message. This broke Step Functions workflows that rely
on error payloads for branching and retry logic.

Now the error includes a JSON object with both the status code and the
original response body:

  {"statusCode":502,"body":"Bad Gateway"}

This allows consumers like Step Functions to inspect the original error
details for decision making.

Fixes #609

Author: bnusunny

Cargo.toml

@@ -20,6 +20,7 @@ exclude = ["examples"]
 bytes = "1.9.0"
 http = "1.2.0"
 http-body = "1.0.1"
+http-body-util = "0.1.0"
 hyper = { version = "1.5.2", features = ["client"] }
 hyper-util = "0.1.10"
 lambda_http = { version = "1.1.1", default-features = false, features = [
@@ -51,7 +52,6 @@ url = "2.5.4"
 [dev-dependencies]
 flate2 = "1.0.25"
 httpmock = "0.8.2"
-http-body-util = "0.1.0"
 http-body = "1.0"
 hyper-rustls = "0.27"
 aws-sigv4 = "1.2.3"

src/lib.rs

@@ -97,6 +97,7 @@ use http::{
     Method, StatusCode,
 };
 use http_body::Body as HttpBody;
+use http_body_util::BodyExt;
 use hyper::body::Incoming;
 use hyper_util::client::legacy::connect::HttpConnector;
 use hyper_util::client::legacy::Client;
@@ -952,9 +953,17 @@ impl Adapter<HttpConnector, Body> {
         if let Some(error_codes) = &self.error_status_codes {
             let status = app_response.status().as_u16();
             if error_codes.contains(&status) {
+                let body_bytes = app_response
+                    .into_body()
+                    .collect()
+                    .await
+                    .map(|c| c.to_bytes())
+                    .unwrap_or_default();
+                let body_str = String::from_utf8_lossy(&body_bytes);
                 return Err(Error::from(format!(
-                    "Request failed with configured error status code: {}",
-                    status
+                    "{{\"statusCode\":{},\"body\":{}}}",
+                    status,
+                    serde_json::to_string(&*body_str).unwrap_or_else(|_| format!("\"{}\"", body_str))
                 )));
             }
         }

tests/integ_tests/main.rs

@@ -740,7 +740,9 @@ async fn test_http_error_status_codes() {
 
     let result = adapter.call(request).await;
     assert!(result.is_err(), "Expected error response for status code 502");
-    assert!(result.unwrap_err().to_string().contains("502"));
+    let err_msg = result.unwrap_err().to_string();
+    assert!(err_msg.contains("502"), "Error should contain status code");
+    assert!(err_msg.contains("Bad Gateway"), "Error should preserve response body");
 
     // Assert endpoint was called
     error_endpoint.assert();