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/RegisterSerialCodeRequest.cs

@@ -0,0 +1,17 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Campaign;
+
+/// <summary>
+/// Body of <c>POST /campaign/regist_serial_code</c>. Client task:
+/// <c>MyPageCodeInputTask</c> (Shadowverse_Code_2026-05-23/Wizard/MyPageCodeInputTask.cs).
+/// </summary>
+[MessagePackObject]
+public sealed class RegisterSerialCodeRequest
+{
+    /// <summary>User-typed serial code. Case-sensitive on the server.</summary>
+    [JsonPropertyName("serial_code")]
+    [Key("serial_code")]
+    public string SerialCode { get; set; } = string.Empty;
+}