feat(user-mypage): UserMyPageController + DTOs + persistence tests

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

SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPage/UserMyPageUpdateResponse.cs

@@ -0,0 +1,12 @@
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.UserMyPage;
+
+/// <summary>
+/// Empty response payload. The client's <c>MyPageSettingUpdateTask.Parse()</c> is the default
+/// pass-through; server just acknowledges.
+/// </summary>
+[MessagePackObject(true)]
+public sealed class UserMyPageUpdateResponse
+{
+}