.NET: [BREAKING] Move AgentSession.Serialize to AIAgent (#3650)

* Move AgentSession.Serialize to AIAgent

* Address PR comments.

* Improve code and fix unit test

* Update test agents to return a default json element instead of throwing where the the result of the serialization is never used.

* Update further tests to actually serialize the session

dotnet/samples/GettingStarted/AgentProviders/Agent_With_CustomImplementation/Program.cs

@@ -31,6 +31,16 @@ namespace SampleApp
         public override ValueTask<AgentSession> CreateSessionAsync(CancellationToken cancellationToken = default)
             => new(new CustomAgentSession());
 
+        public override JsonElement SerializeSession(AgentSession session, JsonSerializerOptions? jsonSerializerOptions = null)
+        {
+            if (session is not CustomAgentSession typedSession)
+            {
+                throw new ArgumentException($"The provided session is not of type {nameof(CustomAgentSession)}.", nameof(session));
+            }
+
+            return typedSession.Serialize(jsonSerializerOptions);
+        }
+
         public override ValueTask<AgentSession> DeserializeSessionAsync(JsonElement serializedSession, JsonSerializerOptions? jsonSerializerOptions = null, CancellationToken cancellationToken = default)
             => new(new CustomAgentSession(serializedSession, jsonSerializerOptions));
 
@@ -136,6 +146,9 @@ namespace SampleApp
 
             internal CustomAgentSession(JsonElement serializedSessionState, JsonSerializerOptions? jsonSerializerOptions = null)
                 : base(serializedSessionState, jsonSerializerOptions) { }
+
+            internal new JsonElement Serialize(JsonSerializerOptions? jsonSerializerOptions = null)
+                => base.Serialize(jsonSerializerOptions);
         }
     }
 }