Add support for URL mode elicitation (#1021)

src/ModelContextProtocol.Core/Protocol/UrlElicitationRequiredErrorData.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the payload for the <c>URL_ELICITATION_REQUIRED</c> JSON-RPC error.
+/// </summary>
+public sealed class UrlElicitationRequiredErrorData
+{
+    /// <summary>
+    /// Gets or sets the elicitations that must be completed before retrying the original request.
+    /// </summary>
+    [JsonPropertyName("elicitations")]
+    public required IReadOnlyList<ElicitRequestParams> Elicitations { get; init; }
+}