Add project files.

Gateway/Models/ProviderResponse.cs

@@ -0,0 +1,16 @@
+namespace Gateway.Models;
+
+public sealed class ProviderResponse
+{
+    public bool Ok { get; set; }
+    public string? RequestId { get; set; }
+    public object? Data { get; set; }
+    public ProviderError? Error { get; set; }
+}
+
+public sealed class ProviderError
+{
+    public string Code { get; set; } = "ERROR";
+    public string Message { get; set; } = "Unknown error";
+    public object? Detail { get; set; }
+}