fix(guild): friend_list returns bare array-at-root (matches GuildFriendListTask.Parse)

GuildFriendListTask.Parse() reads base.ResponseData[data][i] directly —
data must be a bare JSON array, not {friends:[...]}.  Delete the
GuildFriendListResponse wrapper DTO; promote GuildInviteCandidateDto to its
own file.  Controller now returns List<GuildInviteCandidateDto> so the
middleware wraps it as data:[…].  Update wire-shape and integration tests to
assert array-at-root.

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

SVSim.EmulatedEntrypoint/Controllers/GuildController.cs

@@ -174,13 +174,13 @@ public sealed class GuildController : SVSimController
     }
 
     [HttpPost("friend_list")]
-    public async Task<ActionResult<GuildFriendListResponse>> FriendList([FromBody] BaseRequest _, CancellationToken ct)
+    public async Task<ActionResult<List<GuildInviteCandidateDto>>> FriendList([FromBody] BaseRequest _, CancellationToken ct)
     {
         if (!TryGetViewerId(out var viewerId)) return Unauthorized();
 
         var friendInfo = await _friends.GetFriendsAsync(viewerId, ct);
         if (friendInfo.Friends.Count == 0)
-            return new GuildFriendListResponse { Friends = new() };
+            return new List<GuildInviteCandidateDto>();
 
         // For each friend, determine if they are already in a guild (is_join_guild).
         var friendViewerIds = friendInfo.Friends.Select(f => (long)f.ViewerId).ToList();
@@ -206,7 +206,7 @@ public sealed class GuildController : SVSimController
             });
         }
 
-        return new GuildFriendListResponse { Friends = candidates };
+        return candidates;
     }
 
     [HttpPost("invite_user_list")]