.NET: Allow Simulating service stored ChatHistory to improve consistency (#4974)

* Allow Simulating service stored ChatHistory to improve consistency

* Fixing bug in ServiceStoredSimulatingChatClient

* Addressing PR comments.

* Address PR comments

* Apply suggestion from @SergeyMenshykh

Co-authored-by: SergeyMenshykh <68852919+SergeyMenshykh@users.noreply.github.com>

* Fix bug

---------

Co-authored-by: SergeyMenshykh <68852919+SergeyMenshykh@users.noreply.github.com>

dotnet/samples/02-agents/Agents/Agent_Step19_InFunctionLoopCheckpointing/Program.cs

@@ -1,15 +1,16 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 // This sample demonstrates how the ChatClientAgent persists chat history after each individual
-// call to the AI service.
+// call to the AI service, using the SimulateServiceStoredChatHistory option.
 // When an agent uses tools, FunctionInvokingChatClient may loop multiple times
 // (service call → tool execution → service call), and intermediate messages (tool calls and
 // results) are persisted after each service call. This allows you to inspect or recover them
 // even if the process is interrupted mid-loop, but may also result in chat history that is not
 // yet finalized (e.g., tool calls without results) being persisted, which may be undesirable in some cases.
 //
-// To opt into end-of-run persistence instead (atomic run semantics), set
-// PersistChatHistoryAtEndOfRun = true on ChatClientAgentOptions.
+// To use end-of-run persistence instead (atomic run semantics), remove the
+// SimulateServiceStoredChatHistory = true setting (or set it to false). End-of-run
+// persistence is the default behavior.
 //
 // The sample runs two multi-turn conversations: one using non-streaming (RunAsync) and one
 // using streaming (RunStreamingAsync), to demonstrate correct behavior in both modes.
@@ -53,7 +54,7 @@ static string GetTime([Description("The city name.")] string city) =>
         _ => $"{city}: time data not available."
     };
 
-// Create the agent — per-service-call persistence is the default behavior.
+// Create the agent — per-service-call persistence is enabled via SimulateServiceStoredChatHistory.
 // The in-memory ChatHistoryProvider is used by default when the service does not require service stored chat
 // history, so for those cases, we can inspect the chat history via session.TryGetInMemoryChatHistory().
 IChatClient chatClient = string.Equals(store, "TRUE", StringComparison.OrdinalIgnoreCase) ?
@@ -63,6 +64,7 @@ AIAgent agent = chatClient.AsAIAgent(
     new ChatClientAgentOptions
     {
         Name = "WeatherAssistant",
+        SimulateServiceStoredChatHistory = true,
         ChatOptions = new()
         {
             Instructions = "You are a helpful assistant. When asked about multiple cities, call the appropriate tool for each city.",