Add EnsureSuccessAsync method for HTTP error handling

Introduced the `EnsureSuccessAsync` method in `ReCClientHelpers.cs` to handle HTTP response validation asynchronously. This method throws a `ReCApiException` for non-success status codes, including detailed error information such as status code, reason phrase, HTTP method, URI, and response body (if available).

Updated `using` directives to support asynchronous operations and cancellation tokens. Removed redundant `#if NETFRAMEWORK` directive around `using System.Net.Http;` and adjusted `using System.Net.Http.Json;` placement for consistency.

Added exception handling for response body read failures to ensure status information is still propagated. Enhanced error reporting for failed HTTP requests.

src/ReC.Client/ReCClientHelpers.cs

@@ -1,11 +1,10 @@
 using System;
 using System.Globalization;
 using System.Linq;
-using System.Net.Http.Json;
-
-#if NETFRAMEWORK
 using System.Net.Http;
-#endif
+using System.Net.Http.Json;
+using System.Threading;
+using System.Threading.Tasks;
 
 namespace ReC.Client
 {
@@ -45,5 +44,47 @@ namespace ReC.Client
         /// <param name="payload">The payload to serialize.</param>
         /// <returns>A <see cref="JsonContent"/> instance ready for HTTP requests.</returns>
         public static JsonContent ToJsonContent<T>(T payload) => JsonContent.Create(payload);
+
+        /// <summary>
+        /// Throws a <see cref="ReCApiException"/> if the response indicates a non-success status code.
+        /// </summary>
+        /// <param name="response">The HTTP response to inspect.</param>
+        /// <param name="cancel">A token to cancel the operation.</param>
+        public static async Task EnsureSuccessAsync(HttpResponseMessage response, CancellationToken cancel = default)
+        {
+            if (response.IsSuccessStatusCode)
+                return;
+
+#if NETFRAMEWORK
+            string body = null;
+#else
+            string? body = null;
+#endif
+            if (response.Content != null)
+            {
+                try
+                {
+#if NETFRAMEWORK
+                    body = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
+#else
+                    body = await response.Content.ReadAsStringAsync(cancel).ConfigureAwait(false);
+#endif
+                }
+                catch
+                {
+                    // Swallow body read failures; status info is still propagated.
+                }
+            }
+
+            var request = response.RequestMessage;
+            var method = request?.Method?.Method;
+            var uri = request?.RequestUri;
+
+            var message = $"ReC API request failed with status {(int)response.StatusCode} ({response.ReasonPhrase}). "
+                + $"{method} {uri}"
+                + (string.IsNullOrWhiteSpace(body) ? string.Empty : $": {body}");
+
+            throw new ReCApiException(message, response.StatusCode, response.ReasonPhrase, body, method, uri);
+        }
     }
 }