feat: implement CreateMessageBatchAsync method

src/AnthropicClient/Models/MessageBatchRequest.cs

@@ -0,0 +1,28 @@
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a request to create a batch of messages.
+/// </summary>
+public class MessageBatchRequest
+{
+  /// <summary>
+  /// Gets the requests to create messages.
+  /// </summary>
+  public List<MessageBatchRequestItem> Requests { get; init; } = [];
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="MessageBatchRequest"/> class.
+  /// </summary>
+  /// <param name="requests">The requests to create messages.</param>
+  /// <exception cref="ArgumentException">Thrown when <paramref name="requests"/> is empty.</exception>
+  /// <returns>An instance of the <see cref="MessageBatchRequest"/> class.</returns>
+  public MessageBatchRequest(List<MessageBatchRequestItem> requests)
+  {
+    if (requests.Count == 0)
+    {
+      throw new ArgumentException($"{nameof(requests)} must not be empty.", nameof(requests));
+    }
+
+    Requests = requests;
+  }
+}