make all remaining classes with Id implement IIdentifiable: Agent and…

… GameEvent

src/game-lib/Events/GameEvent.cs

@@ -1,8 +1,10 @@
-namespace UfoGameLib.Events;
+using Lib.Json;
 
-public abstract class GameEvent
+namespace UfoGameLib.Events;
+
+public abstract class GameEvent : IIdentifiable
 {
-    public readonly int Id;
+    public int Id { get; }
     public readonly string Type;
 
     public GameEvent(int id, string type)

src/game-lib/Model/Agent.cs

@@ -1,13 +1,10 @@
 using System.Text.Json.Serialization;
 using Lib.Contracts;
+using Lib.Json;
 
 namespace UfoGameLib.Model;
 
-// kja-wishlist I think that yes, it should be IIdentifiable. lib\Json\JsonConverterSupportingReferences.cs:86
-// should this implement IIdentifiable the same as MissionSite?
-// I think currently it doesn't because Agents are never serialized by reference
-// by GameStateJsonConverter, so it doesn't matter if they implement IIdentifiable
-public class Agent
+public class Agent : IIdentifiable
 {
     public enum AgentState
     {
@@ -21,7 +18,7 @@ public enum AgentState
         Terminated
     }
 
-    public readonly int Id;
+    public int Id { get; }
 
     public readonly int TurnHired;
     public AgentState CurrentState;