Use descriptive error message for timed out requests (#2356)

When a request times out, ErrorMessage was set to the TaskCanceledException
message ("A task was canceled.") which contradicts ResponseStatus.TimedOut.
Now ErrorMessage says "The request timed out." for timeout scenarios, keeping
all three state properties (ResponseStatus, ErrorMessage, ErrorException)
consistent. ErrorException still holds the original TaskCanceledException.

Fixes #2257

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

src/RestSharp/RestClient.Async.cs

@@ -61,11 +61,13 @@ public async Task<RestResponse> ExecuteAsync(RestRequest request, CancellationTo
     }
 
     static RestResponse GetErrorResponse(RestRequest request, Exception exception, CancellationToken timeoutToken) {
+        var timedOut = exception is OperationCanceledException && TimedOut();
+
         var response = new RestResponse(request) {
             ResponseStatus = exception is OperationCanceledException
-                ? TimedOut() ? ResponseStatus.TimedOut : ResponseStatus.Aborted
+                ? timedOut ? ResponseStatus.TimedOut : ResponseStatus.Aborted
                 : ResponseStatus.Error,
-            ErrorMessage   = exception.GetBaseException().Message,
+            ErrorMessage   = timedOut ? "The request timed out." : exception.GetBaseException().Message,
             ErrorException = exception
         };