.NET: Allow Simulating service stored ChatHistory to improve consistency

[westey-m]

Motivation and Context

When storing chat history via ChatHistoryProvider has some differences to storing chat history in the underlying AI service.
This PR introduces a way to use a ChatHistoryProvider but with a ChatClient decorator that simulates storing chat history in the underlying AI service, so that their behavior matches.

#4976

Description

• Allow Simulating service stored ChatHistory to improve consistency

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR introduces an opt-in mode for ChatClientAgent to simulate service-stored chat history while still using a ChatHistoryProvider, aiming to make framework-managed persistence behave like service-managed history during multi-call function/tool loops.

Changes:

• Add ServiceStoredSimulatingChatClient decorator and inject it when ChatClientAgentOptions.SimulateServiceStoredChatHistory is enabled.
• Replace/remove the previous ChatHistoryPersistingChatClient approach and update agent logic + tests to match the new default (end-of-run) vs opt-in (per-service-call) behavior.
• Update samples and ADR documentation to reflect the new persistence model and configuration matrix.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ServiceStoredSimulatingChatClient.cs	New decorator that loads history before each call and persists after each call while returning a sentinel conversation id.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatHistoryPersistingChatClient.cs	Removed prior per-service-call/mark-only persistence decorator.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs	Conditionally inject the new decorator when SimulateServiceStoredChatHistory is enabled.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientBuilderExtensions.cs	Replace old builder extension with UseServiceStoredChatHistorySimulation.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentOptions.cs	Replace PersistChatHistoryAtEndOfRun with SimulateServiceStoredChatHistory and update docs/clone.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentLogMessages.cs	Update warnings/messages to reference the new option/decorator.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Adjust history loading/persistence control flow to account for simulation mode and add shared LoadChatHistoryAsync.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ServiceStoredSimulatingChatClientTests.cs	Rename/repurpose tests to cover the new decorator/option behavior.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatOptionsMergingTests.cs	Update expectations: inner client receives null ChatOptions when none are provided.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatHistoryManagementTests.cs	Update test options to use SimulateServiceStoredChatHistory.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ApprovalsTests.cs	Update approval-flow tests to reference new decorator/option.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Update expectations around ChatOptions forwarding.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTestHelper.cs	Update comments/refs to new decorator.
dotnet/samples/02-agents/Agents/Agent_Step19_InFunctionLoopCheckpointing/README.md	Update sample explanation to use SimulateServiceStoredChatHistory.
dotnet/samples/02-agents/Agents/Agent_Step19_InFunctionLoopCheckpointing/Program.cs	Enable SimulateServiceStoredChatHistory in the sample.
docs/decisions/0022-chat-history-persistence-consistency.md	Update ADR to reflect opt-in simulation approach and new configuration matrix.

Comments suppressed due to low confidence (2)

dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ServiceStoredSimulatingChatClientTests.cs:54

• This test’s summary/name say it verifies the default behavior when SimulateServiceStoredChatHistory is false, but the test explicitly sets SimulateServiceStoredChatHistory = true. Either update the wording/name to reflect the opt-in behavior, or remove the explicit setting and adjust assertions to match the real default (end-of-run persistence).
  dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ServiceStoredSimulatingChatClientTests.cs:115
• RunAsync_PersistsMessagesAtEndOfRun_WhenOptionEnabledAsync is now configured with SimulateServiceStoredChatHistory=true, but the test comment/assert description claims it’s verifying end-of-run persistence. With SimulateServiceStoredChatHistory enabled, ChatClientAgent skips end-of-run persistence unless forced (continuation/background), so this test no longer validates what its name suggests and could mask regressions. Consider rewriting it to cover the actual default end-of-run persistence case (SimulateServiceStoredChatHistory=false), or to explicitly assert the decorator is the component performing persistence when simulation is enabled.