feat(friend): wire DTOs for /friend/* endpoints

12 files: 3 entry types (FriendEntryDto, FriendApplyEntryDto,
PlayedTogetherEntryDto), 5 response wrappers, 4 request DTOs.
All carry [MessagePackObject] + [Key] + [JsonPropertyName] per convention.

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

SVSim.EmulatedEntrypoint/Models/Dtos/Friend/SearchUserResponse.cs

@@ -0,0 +1,15 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Friend;
+
+/// <summary>
+/// Search result. When no match, <see cref="UserInfo"/> serializes as <c>{}</c>
+/// (an empty JSON object). When matched, it's a populated <see cref="FriendEntryDto"/>.
+/// </summary>
+[MessagePackObject]
+public sealed class SearchUserResponse
+{
+    [JsonPropertyName("user_info")][Key("user_info")]
+    public object UserInfo { get; set; } = new();
+}