feat(campaign): CampaignController.RegisterSerialCode + DTOs + tests

Adds POST /campaign/regist_serial_code with 9 integration tests covering
success path, all disqualifier conditions (unknown/disabled/expired/future/
already-redeemed/unsupported-reward-type), 401 on missing auth, and case-
sensitivity. IsSupportedGiftRewardType uses gift-wire literals (1/4/9) not
UserGoodsType enum values, matching GiftController.WireRewardTypeToUserGoodsType.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Campaign/RegisterSerialCodeResponse.cs

@@ -0,0 +1,16 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Campaign;
+
+/// <summary>
+/// Success response shape. Failure path uses an anonymous <c>{ result_code = 4202 }</c>
+/// (mirroring AchievementController/MissionController) and bypasses this DTO.
+/// </summary>
+[MessagePackObject]
+public sealed class RegisterSerialCodeResponse
+{
+    [JsonPropertyName("is_complete")]
+    [Key("is_complete")]
+    public bool IsComplete { get; set; }
+}