Add ReCApiException class for API error handling

A new `ReCApiException` class was introduced in the `ReC.Client` namespace to represent errors returned by the ReC API.

The class includes properties for detailed error information:
- `StatusCode`, `ReasonPhrase`, `ResponseBody`, `Method`, and `RequestUri`.

A constructor was added to initialize these properties. Conditional compilation directives ensure compatibility between .NET Framework and other .NET targets. The class is marked as `[Serializable]` for non-.NET Framework targets.

src/ReC.Client/ReCApiException.cs

@@ -0,0 +1,88 @@
+using System;
+using System.Net;
+using System.Net.Http;
+
+namespace ReC.Client
+{
+    /// <summary>
+    /// Represents an error returned by the ReC API.
+    /// </summary>
+#if !NETFRAMEWORK
+    [Serializable]
+#endif
+    public class ReCApiException : Exception
+    {
+        /// <summary>
+        /// The HTTP status code returned by the API.
+        /// </summary>
+        public HttpStatusCode StatusCode { get; }
+
+        /// <summary>
+        /// The HTTP reason phrase returned by the API, if any.
+        /// </summary>
+#if NETFRAMEWORK
+        public string ReasonPhrase { get; }
+#else
+        public string? ReasonPhrase { get; }
+#endif
+
+        /// <summary>
+        /// The raw response body returned by the API, if any.
+        /// </summary>
+#if NETFRAMEWORK
+        public string ResponseBody { get; }
+#else
+        public string? ResponseBody { get; }
+#endif
+
+        /// <summary>
+        /// The HTTP method used for the failed request.
+        /// </summary>
+#if NETFRAMEWORK
+        public string Method { get; }
+#else
+        public string? Method { get; }
+#endif
+
+        /// <summary>
+        /// The request URI that was called.
+        /// </summary>
+#if NETFRAMEWORK
+        public Uri RequestUri { get; }
+#else
+        public Uri? RequestUri { get; }
+#endif
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ReCApiException"/> class.
+        /// </summary>
+        /// <param name="message">A summary message describing the error.</param>
+        /// <param name="statusCode">The HTTP status code returned by the API.</param>
+        /// <param name="reasonPhrase">The HTTP reason phrase returned by the API.</param>
+        /// <param name="responseBody">The raw response body returned by the API.</param>
+        /// <param name="method">The HTTP method used for the request.</param>
+        /// <param name="requestUri">The request URI that was called.</param>
+        public ReCApiException(
+            string message,
+            HttpStatusCode statusCode,
+#if NETFRAMEWORK
+            string reasonPhrase,
+            string responseBody,
+            string method,
+            Uri requestUri
+#else
+            string? reasonPhrase,
+            string? responseBody,
+            string? method,
+            Uri? requestUri
+#endif
+            ) : base(message)
+        {
+            StatusCode = statusCode;
+            ReasonPhrase = reasonPhrase;
+            ResponseBody = responseBody;
+            Method = method;
+            RequestUri = requestUri;
+        }
+    }
+}