Make UrlElicitationRequiredErrorData.Elicitations consistent with other Protocol DTOs (#1335)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/UrlElicitationRequiredErrorData.cs

@@ -12,5 +12,5 @@ public sealed class UrlElicitationRequiredErrorData
     /// Gets or sets the elicitations that must be completed before retrying the original request.
     /// </summary>
     [JsonPropertyName("elicitations")]
-    public required IReadOnlyList<ElicitRequestParams> Elicitations { get; init; }
+    public required IList<ElicitRequestParams> Elicitations { get; set; }
 }

src/ModelContextProtocol.Core/UrlElicitationRequiredException.cs

@@ -12,7 +12,7 @@ namespace ModelContextProtocol;
 /// </summary>
 public sealed class UrlElicitationRequiredException : McpProtocolException
 {
-    private readonly IReadOnlyList<ElicitRequestParams> _elicitations;
+    private readonly List<ElicitRequestParams> _elicitations;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="UrlElicitationRequiredException"/> class with the specified message and pending elicitations.
@@ -66,7 +66,7 @@ internal static bool TryCreateFromError(
         return true;
     }
 
-    private static bool TryParseElicitations(JsonElement dataElement, out IReadOnlyList<ElicitRequestParams> elicitations)
+    private static bool TryParseElicitations(JsonElement dataElement, out IList<ElicitRequestParams> elicitations)
     {
         elicitations = [];
 
@@ -93,7 +93,7 @@ private static bool TryParseElicitations(JsonElement dataElement, out IReadOnlyL
         return true;
     }
 
-    private static IReadOnlyList<ElicitRequestParams> Validate(IEnumerable<ElicitRequestParams> elicitations)
+    private static List<ElicitRequestParams> Validate(IEnumerable<ElicitRequestParams> elicitations)
     {
         var list = new List<ElicitRequestParams>();
         foreach (var elicitation in elicitations)