feat(account): add /account/update_name endpoint

Implements POST /account/update_name — writes Viewer.DisplayName and
returns an empty array per the prod capture. Includes TDD test covering
the persist side-effect.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Account/AccountUpdateNameRequest.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Account;
+
+[MessagePackObject]
+public class AccountUpdateNameRequest : BaseRequest
+{
+    [JsonPropertyName("name")]
+    [Key("name")]
+    public string Name { get; set; } = string.Empty;
+}