Deck list work

SVSim.EmulatedEntrypoint/Models/Dtos/SpecialDeckFormat.cs

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+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos;
+
+/// <summary>
+/// One entry under /mypage/index data.room_type_in_session.special_deck_format_list. Consumed by
+/// RoomRuleInfo ctor (Wizard/RoomRuleInfo.cs:61-70) which is TryGetValue-guarded but the per-entry
+/// fields are accessed unconditionally inside the guard.
+///
+/// Prod-captured shape: <c>{"deck_format":"5","end_time":"2030-06-26 19:59:59"}</c>.
+/// </summary>
+[MessagePackObject]
+public class SpecialDeckFormat
+{
+    /// <summary>Wire is string per prod's PHP convention (despite looking numeric like "5").</summary>
+    [JsonPropertyName("deck_format")]
+    [Key("deck_format")]
+    public string DeckFormat { get; set; } = string.Empty;
+
+    /// <summary>"yyyy-MM-dd HH:mm:ss" wire format.</summary>
+    [JsonPropertyName("end_time")]
+    [Key("end_time")]
+    public string EndTime { get; set; } = string.Empty;
+}